Arduino UNO with HC-06 bluetooth module stoped working all of a sudden

Hello!
My goal was to send data to an Arduino by a self programmed Android app. To prevent any personal fault, i took firstly a app and the matching Sketch from a Youtube Project. The App/Sketch should turn a LED on and off. Because I tried to connect my Mega 2560 (unsuccesfully) a day before I changed some lines but it was basicly the same code.

Here is now was happend:

  • I upload my changed sketch and it worked almost perfect. The readString was depending on the pressed button "on" or "off" and it was printed via the serial monitor.
  • I made some minor changes because I added unnecessary lines the day before to troubleshoot my MEGA
  • I recogognized after a few tweaks and uploads that I had the "getsync(): not in sync resp=0x30" error all the time. So I didn't upload at all.
  • I pull the HC-06 VCC off and upload succesfully the orignial youtube sketch -> up this point i had no more transmission

The current state:

  • The Android device and the HC-06 can connect, I just can't receive any data. The TX and RX light's aren't blinking anymore if I press a button on my app.

So my Questions are:

  • Have I already broken the module or my UNO r3? And if yes, do got any tips for me to prevent that the next time?
  • Is it possible that I deleted something important from the sketch? (Although I tried the fully functional youtube sketch several times after that)
  • Do i got an error in my Setup?

Sketch:

int ledPin = 13;
String readString;

void setup() {
Serial.begin(9600);
pinMode(ledPin, OUTPUT);
}

void loop() {
while (Serial.available()) {
delay(3);
char c = Serial.read();
readString += c;
}
if (readString.length() >0) {
Serial.println(readString);
if (readString == "on")
{
digitalWrite(ledPin, HIGH);
}
if (readString == "off")
{
digitalWrite(ledPin, LOW);
}
readString="";
}
}

Youtube source: https://www.youtube.com/watch?v=6o_QVlltNgM

my Setup: Arduino Sainsmart UNO r3
HC-06
VCC -> 5V
GND -> GND
RXD -> RX
TXD -> TX

thanks in advance