wvmarle:
You probably got a stack trace with it, use the ESP Exception Decoder to get readable info out of that. There's probably an error in your code.No idea about the AT commands, using the ESP as standalone processor myself. You should be able to upload another sketch (my favourite test sketch is Blink, to blink the internal blue LED at GPIO2).
I tried connecting GPIO0 to GND again but it gives the same error... How do I reset the ESP8266?