egzi:
First problem appeared when I was playing with it, I called AT+COPN command, it started to flood the screen of command terminal with operator names in a infinity loop then crashed. When powered on it showed some startup responses, some random characters and it didn't want to respond to any commands. I tried different things with it and sometimes I managed to start it up properly. After some time it started to work again.
Overall it still sends some random characters from time to time or hangs.
I had the same problem today. I tried the AT+COPN command and it gave me an infinite loop and after I reset the module I had exactly the same problems like you. Was pretty much unuseable, kinda like bricked. I solved the problem by doing a factory reset. After this was done everything is working again.
First power up the module and immediately flood it with the"AT" command till it responds. Then call AT&F or AT&F0 - one of those worked for me. Baudrate is reset to 115200.