I was listening to music in the car, and my iPhone 3GS just turned off. It went onto the black screen, and didn't turn on (It had about 80% battery). When I plugged it in when I got home, there was no response from it, same when in iTunes in 2 computers 1 after the other.
I have had it fully submerged in water, but the only thing that happened from that was the home button stopped working, so I know use assistive touch, meaning I can't put it in DFU as far as I know of. This was about a month ago and I haven't had any other effects from that so far.
Have you tried hard restarting it? Holding down home button and sleep/wake button until Apple logo appears? You can manually put phone into recovery mode by having it plugged in, hold down sleep /wake button and home button until Apple logo appears. Once it does, let go of sleep / wake button and continue to hold home button until iTunes gives pop saying it found phone in recovery mode, restore to continue.