From the manual:
If the FAULT light is lit yet the PLC operates correctly and the RUN light is lit, replace the battery.
If the FAULT light is lit and the RUN light is off there are several other possibilities including I/O module failure, power supply failure, memory failure, CPU failure. As you can see, just about anything.
I know a little about this PLC. I have a programming manual that was scanned and saved to a PDF file. It you would like a copy, please send me a private message with an e-mail address. It is almost 6 megs so I cant post it here. If you just need to recover the program I might be able to help you with that also.