can't use i2c device

Hello, i've been working on getting a gy-521 board (mpu6050) to communicate with my Arduino. I've done this multiple times before, but now it seems not to work.
The connections are:
Arduino gy-521
Vcc --------Vcc
Gnd--------Gnd
SDA--------SDA
SCL---------SCL

Ive tried it on three different gy-521's and all three of them don't work
This is my code:

//Include LCD and I2C library
//#include <LiquidCrystal_I2C.h>
#include <Wire.h>

//Declaring some global variables
int gyro_x, gyro_y, gyro_z;
long acc_x, acc_y, acc_z, acc_total_vector;
int temperature;
long gyro_x_cal, gyro_y_cal, gyro_z_cal;
long loop_timer;
int lcd_loop_counter;
float angle_pitch, angle_roll;
int angle_pitch_buffer, angle_roll_buffer;
boolean set_gyro_angles;
float angle_roll_acc, angle_pitch_acc;
float angle_pitch_output, angle_roll_output;

void setup() {
  Wire.begin();                                                        //Start I2C as master
  //Serial.begin(57600);                                               //Use only for debugging
  pinMode(13, OUTPUT);    
  Serial.begin(115200);    //Initialize the LCD
  Serial.println("begin");//Set output 13 (LED) as output
  
  setup_mpu_6050_registers();                                          //Setup the registers of the MPU-6050 (500dfs / +/-8g) and start the gyro
  Serial.println("registers set");
  digitalWrite(13, HIGH);                                              //Set digital output 13 high to indicate startup

  for (int cal_int = 0; cal_int < 2000 ; cal_int ++){                  //Run this code 2000 times
    if(cal_int % 125 == 0) Serial.print("*");                             //Print a dot on the LCD every 125 readings
    read_mpu_6050_data();                                              //Read the raw acc and gyro data from the MPU-6050
    gyro_x_cal += gyro_x;                                              //Add the gyro x-axis offset to the gyro_x_cal variable
    gyro_y_cal += gyro_y;                                              //Add the gyro y-axis offset to the gyro_y_cal variable
    gyro_z_cal += gyro_z;                                              //Add the gyro z-axis offset to the gyro_z_cal variable
    delay(3);                                                          //Delay 3us to simulate the 250Hz program loop
  }
  gyro_x_cal /= 2000;                                                  //Divide the gyro_x_cal variable by 2000 to get the avarage offset
  gyro_y_cal /= 2000;                                                  //Divide the gyro_y_cal variable by 2000 to get the avarage offset
  gyro_z_cal /= 2000;                                                  //Divide the gyro_z_cal variable by 2000 to get the avarage offset

  /*lcd.clear();                                                         //Clear the LCD
  
  lcd.setCursor(0,0);                                                  //Set the LCD cursor to position to position 0,0
  lcd.print("Pitch:");                                                 //Print text to screen
  lcd.setCursor(0,1);                                                  //Set the LCD cursor to position to position 0,1
  lcd.print("Roll :");   */                                              //Print text to screen
  
  digitalWrite(13, LOW);                                               //All done, turn the LED off
  
//  loop_timer = micros();                                               //Reset the loop timer
}

void loop(){

  read_mpu_6050_data();                                                //Read the raw acc and gyro data from the MPU-6050

  gyro_x -= gyro_x_cal;                                                //Subtract the offset calibration value from the raw gyro_x value
  gyro_y -= gyro_y_cal;                                                //Subtract the offset calibration value from the raw gyro_y value
  gyro_z -= gyro_z_cal;                                                //Subtract the offset calibration value from the raw gyro_z value
  
  //Gyro angle calculations
  //0.0000611 = 1 / (250Hz / 65.5)
  angle_pitch += gyro_x * 0.0000611;                                   //Calculate the traveled pitch angle and add this to the angle_pitch variable
  angle_roll += gyro_y * 0.0000611;                                    //Calculate the traveled roll angle and add this to the angle_roll variable
  
  //0.000001066 = 0.0000611 * (3.142(PI) / 180degr) The Arduino sin function is in radians
  angle_pitch += angle_roll * sin(gyro_z * 0.000001066);               //If the IMU has yawed transfer the roll angle to the pitch angel
  angle_roll -= angle_pitch * sin(gyro_z * 0.000001066);               //If the IMU has yawed transfer the pitch angle to the roll angel
  
  //Accelerometer angle calculations
  acc_total_vector = sqrt((acc_x*acc_x)+(acc_y*acc_y)+(acc_z*acc_z));  //Calculate the total accelerometer vector
  //57.296 = 1 / (3.142 / 180) The Arduino asin function is in radians
  angle_pitch_acc = asin((float)acc_y/acc_total_vector)* 57.296;       //Calculate the pitch angle
  angle_roll_acc = asin((float)acc_x/acc_total_vector)* -57.296;       //Calculate the roll angle
  
  //Place the MPU-6050 spirit level and note the values in the following two lines for calibration
  angle_pitch_acc -= -0.6;                                              //Accelerometer calibration value for pitch
  angle_roll_acc -= -2.7;                                               //Accelerometer calibration value for roll

  if(set_gyro_angles){                                                 //If the IMU is already started
    angle_pitch = angle_pitch * 0.9996 + angle_pitch_acc * 0.0004;     //Correct the drift of the gyro pitch angle with the accelerometer pitch angle
    angle_roll = angle_roll * 0.9996 + angle_roll_acc * 0.0004;        //Correct the drift of the gyro roll angle with the accelerometer roll angle
  }
  else{                                                                //At first start
    angle_pitch = angle_pitch_acc;                                     //Set the gyro pitch angle equal to the accelerometer pitch angle 
    angle_roll = angle_roll_acc;                                       //Set the gyro roll angle equal to the accelerometer roll angle 
    set_gyro_angles = true;                                            //Set the IMU started flag
  }
  
  //To dampen the pitch and roll angles a complementary filter is used
  angle_pitch_output = angle_pitch_output * 0.9 + angle_pitch * 0.1;   //Take 90% of the output pitch value and add 10% of the raw pitch value
  angle_roll_output = angle_roll_output * 0.9 + angle_roll * 0.1;      //Take 90% of the output roll value and add 10% of the raw roll value
  
  Serial.print("Pitch: ");      
  Serial.print(angle_pitch_output); 
  Serial.print("  roll: "); 
  Serial.print(angle_roll_output); //Write the roll and pitch values to the LCD display
  Serial.print("Pitch: ");      
  Serial.print(gyro_x); 
  Serial.print("  roll: "); 
  Serial.println(gyro_y); //Write the roll and pitch values to the LCD display

}

I also tried an I2c scan

#include <Wire.h>
 
 
void setup()
{
  Wire.begin();
 
  Serial.begin(9600);
  while (!Serial);             // Leonardo: wait for serial monitor
  Serial.println("\nI2C Scanner");
}
 
 
void loop()
{
  byte error, address;
  int nDevices;
 
  Serial.println("Scanning...");
 
  nDevices = 0;
  for(address = 1; address < 127; address++ )
  {
    // The i2c_scanner uses the return value of
    // the Write.endTransmisstion to see if
    // a device did acknowledge to the address.
    Wire.beginTransmission(address);
    error = Wire.endTransmission();
 
    if (error == 0)
    {
      Serial.print("I2C device found at address 0x");
      if (address<16)
        Serial.print("0");
      Serial.print(address,HEX);
      Serial.println("  !");
 
      nDevices++;
    }
    else if (error==4)
    {
      Serial.print("Unknown error at address 0x");
      if (address<16)
        Serial.print("0");
      Serial.println(address,HEX);
    }    
  }
  if (nDevices == 0)
    Serial.println("No I2C devices found\n");
  else
    Serial.println("done\n");
 
  delay(5000);           // wait 5 seconds for next scan
}

but it scanned forever and didn't return anything, even when I disconnected the device

That code doesn't compile.

Are the GY-521 board from different suppliers or from the same Chinese website?

that code comes from Joop Brokking and you should really credit him for that.
(do you have an LCD display connected also ?)

if you really just want to confirm the MPU-6050 is working and only want to see raw data coming through, go to the Arduino Playground and use the simple sketch from there.