It is best to try the steps below.
Try to do a CMOS clear of the board.
Put the bios jumper to the maintenance mode.
Boot the system with minimum configuration (the rest disconnected)
PSU + Motherboard + CPU + 1 Memory Module only.
Put back the jumper to default position when done.
If it is not booting, remove the memory to check if the board issues any beeps
Then last thing to do is to recover the bios using the Recovery Method
If you have any other CPU, test it.
I had a 915GAG that did that right after the warranty went out. It started shutting down randomly. Ran Memtest (freeware can download it off the internet) on it and found 1 bad module. Tossed that and the issues kept happening. Eventually it just stopped booting all together. I pulled the CPU and used it in an ASUS P5KPL-CM board (since Intel didn't have a current board at the time for the CPU). That was last March and so far the computer is running well.