No signal

Yesterday i had to fly at a wind power generator. First fly went good, but during the second
I had NO SIGNAL on the GO app. I thought i lost control, but I could land
manually. Switching on, off, changing batterys and restarting the tablet did not
fix it. When I looked through the menues of the app i found a message about the compass. ..
There was something metall and it need to be calibrated again. ..
I did it and everything was fine.

Any idea what happens ?