Problems with my car?

As I was driving, the engine auto-diagnosis light came on and stayed on for approximately three minutes, ending with the engine cutting out. I called the AA, the AA man's exact words to me were "there was no spark". He said that it was the electrics; he took something off the car and replaced it with something else. He told me to turn on the engine, but the car did not start up. All the lights on the dashboard were out at this point. Then he fiddled with the battery. I noticed that the battery light was flashing, so, I asked the man about it and he told me not to worry. The AA man told me that he could not fix it and he towed me to a garage near my home. The battery light had now stopped flashing but was still on.

The next morning, I went back to the garage and they said they would have a look at it; this was a week ago. Since then I have been told by the garage that they cannot find the source of the problem. The mechanic asked me to bring over my spare keys and while I was there I got into the car, I noticed that the battery light was still on. I turned the ignition, the engine turned but the car didn't start.

I spoke to the mechanic and he told me that he thinks that it would cost me £1500-2000 to fix the car because "a sensor on the car wasn't sending signals for the car to start up".

He then said that my headlights and other lights were not working in my car anymore. Could this be to do with the battery light being on all week? My car is a CITROEN C3 LX.

If anybody as an idea of what the problem might be, please let me know.