reset stepper motor position

Hi, first of all I apologize for my English, (I translate with google translator), I would like to submit a small, big problem. I run a stepper motor with Arduino Mega to place a cart by entering the measurement with keypad. The problem is that when I turn off the machine, clearly the carriage is in position 0 and so on power is a problem. I wanted to know if changing the code you can add a loop and a condition so that "order" to Arduino, the power of the same, with the push of a dedicated button, to reset the engine. Or to the engine button is pressed will step back up to find a sensor to "zero" and then create the ideal condition to start the second loop. I ask for help for my limits in the knowledge of the subject.
Thank you.
Domenico greetings

#include <AccelStepper.h> // AccelStepper Library
#include <Keypad.h>  // Keypad Library
#include "U8glib.h"  // U8glib for Nokia LCD

// Variables to hold entered number on Keypad
volatile int firstnumber=99;  // used to tell how many numbers were entered on keypad
volatile int secondnumber=99;
volatile int thirdnumber=99;
volatile int fourthnumber=99;

// Variables to hold Distance and CurrentPosition
int keyfullnumber=0;  // used to store the final calculated distance value
String currentposition = "";  // Used for display on Nokia LCD


// Keypad Setup
const byte ROWS = 4; // Four Rows
const byte COLS = 4; // Four Columns
char keys[ROWS][COLS] = {
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};
byte rowPins[ROWS] = {22, 24, 26, 28}; // Arduino pins connected to the row pins of the keypad
byte colPins[COLS] = {31, 33, 35, 37}; // Arduino pins connected to the column pins of the keypad
Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );  // Keypad Library definition


// U8glib Setup for Nokia LCD
#define backlight_pin 11
U8GLIB_PCD8544 u8g(3, 4, 6, 5, 7);  // Arduino pins connected to Nokia pins:
                                    // CLK=3, DIN=4, CE=6, DC=5, RST=7
                                    
                                    
// AccelStepper Setup
AccelStepper stepper(1, A0, A1);  // 1 = Easy Driver interface
                                  // Arduino A0 connected to STEP pin of Easy Driver
                                  // Arduino A1 connected to DIR pin of Easy Driver
                                  


void setup(void) {
  
  //  Light up the LCD backlight LEDS
  analogWrite(backlight_pin, 0);  // Set the Backlight intensity (0=Bright, 255=Dim)
    
  //  AccelStepper speed and acceleration setup
  stepper.setMaxSpeed(3000);  // Not to fast or you will have missed steps
  stepper.setAcceleration(400);  //  Same here
  
  // Draw starting screen on Nokia LCD
  u8g.firstPage();
  do {
  u8g.drawHLine(0, 15, 84);
  u8g.drawVLine(50, 16, 38);
  u8g.drawHLine(0, 35, 84); 
  u8g.setFont(u8g_font_profont11);
  u8g.drawStr(0, 10, "DIGITA MISURA");
  u8g.drawStr(62, 29, "MM");
  u8g.drawStr(4, 46, "MISURA");
  }
  while( u8g.nextPage() );
  
}


void loop(){
  
  char keypressed = keypad.getKey();  // Get value of keypad button if pressed
  if (keypressed != NO_KEY){  // If keypad button pressed check which key it was
    switch (keypressed) {
      
      case '1':
        checknumber(1);
      break;
        
      case '2':
        checknumber(2);
      break;

      case '3':
        checknumber(3);
      break;

      case '4':
        checknumber(4);
      break;

      case '5':
        checknumber(5);
      break;

      case '6':
        checknumber(6);
      break;

      case '7':
        checknumber(7);
      break;

      case '8':
        checknumber(8);
      break;

      case '9':
        checknumber(9);
      break;

      case '0':
        checknumber(0);
      break;

      case '*':
        deletenumber();
      break;

      case '#':
        calculatedistance();
      break;
    }
  }

}

void checknumber(int x){
  if (firstnumber == 99) { // Check if this is the first number entered
    firstnumber=x;
    String displayvalue = String(firstnumber);  //  Transform int to a string for display
    drawnokiascreen(displayvalue); // Redraw Nokia lcd
    
  } else {
    if (secondnumber == 99) {  // Check if it's the second number entered
      secondnumber=x;
      String displayvalue = (String(firstnumber) + String(secondnumber));
      drawnokiascreen(displayvalue);
  } else {
    if (thirdnumber == 99) {  // Check if it's the 3rd number entered
      thirdnumber=x;
      String displayvalue = (String(firstnumber) + String(secondnumber) + String(thirdnumber));
      drawnokiascreen(displayvalue);

    } else {  // It must be the 3rd number entered
      fourthnumber=x;
      String displayvalue = (String(firstnumber) + String(secondnumber) + String(thirdnumber) + String(fourthnumber));
      drawnokiascreen(displayvalue);

      }
    }
  }
}

 
void deletenumber() {  // Used to backspace entered numbers
    if (fourthnumber !=99) {
      String displayvalue = (String(firstnumber) + String(secondnumber) + String(fourthnumber));
      drawnokiascreen(displayvalue);

    fourthnumber=99;
  } 
  else {  
   if (thirdnumber !=99) {
      String displayvalue = (String(firstnumber) + String(secondnumber));
      drawnokiascreen(displayvalue);

    thirdnumber=99;
  } 
  else {
    if (secondnumber !=99) {
      String displayvalue = String(firstnumber);
      drawnokiascreen(displayvalue);

      secondnumber=99;
   } 
   else {
     if (firstnumber !=99) {
       String displayvalue = "";
       drawnokiascreen(displayvalue);

       firstnumber=99;
        }
      }
    }
  }
}
  
void calculatedistance() {  // Used to create a full number from entered numbers

    if (fourthnumber == 99 && thirdnumber == 99 && secondnumber == 99 && firstnumber != 99) {
      keyfullnumber=firstnumber;
      movestepper(keyfullnumber);
    }
    
    if (secondnumber != 99 && thirdnumber == 99 && fourthnumber == 99) {
      keyfullnumber=(firstnumber*10)+secondnumber;
      movestepper(keyfullnumber);
    }
    
    if (thirdnumber != 99 && fourthnumber == 99) {
      keyfullnumber=(firstnumber*100)+(secondnumber*10)+thirdnumber;
      movestepper(keyfullnumber);
    }
        
    if (fourthnumber != 99) {
      keyfullnumber=(firstnumber*1000)+(secondnumber*100)+(thirdnumber*10)+fourthnumber;
      movestepper(keyfullnumber);
    }
    
    resetnumbers(); // Reset numbers to get ready for new entry
  } 


void movestepper(int z) {  //  Move the stepper

  long calculatedmove=(z*40L);  //  Calculate number of steps needed in mm
  stepper.runToNewPosition(calculatedmove);
  currentposition = String(z);
  u8g.firstPage();
  do {
    u8g.drawHLine(0, 15, 84);
    u8g.drawVLine(50, 16, 38);
    u8g.drawHLine(0, 35, 84); 
    u8g.setFont(u8g_font_profont11);
    u8g.drawStr(0, 10, "DIGITA MISURA");
    u8g.drawStr(62, 29, "MM");
    u8g.drawStr(4, 46, "MISURA");
    u8g.setPrintPos(57,47);
    u8g.print(currentposition);       
  }
  while( u8g.nextPage() ); 
}
                
void resetnumbers() {  // Reset numbers for next entry
  firstnumber=99;
  secondnumber=99;
  thirdnumber=99;
  fourthnumber=99;
} 
  

void drawnokiascreen(String y) {

    u8g.firstPage();
    do {
      u8g.drawHLine(0, 15, 84);
      u8g.drawVLine(50, 16, 38);
      u8g.drawHLine(0, 35, 84); 
      u8g.setFont(u8g_font_profont11);
      u8g.drawStr(0, 10, "DIGITA MISURA");
      u8g.setPrintPos(0,29);
      u8g.print(y);  // Put entered number on Nokia lcd    
      u8g.drawStr(62, 29, "MM");
      u8g.drawStr(4, 46, "MISURA");
      u8g.setPrintPos(57,47); 
      u8g.print(currentposition);  //  Display current position of stepper
    }
      while( u8g.nextPage() );

}

Most people use a limit switch to set the zero or home position.

Every time the Arduino is turned ON, it drives the motor until the limit switch is activated, and that becomes zero.

So much for the hardware, but in terms of code as I do?

while(1) {
   step_backward(); //or forward, if appropriate
   if( digitalRead(limitPin) == LOW) break; //or HIGH, if appropriate
}
  motorpos = 0;  //set zero position