Try removing all components but CPU and CPU fan and start the board. If the CPU fan is spinning when starting the board, you should get 1 long 2 short beeps.
Try also clearing the CMOS (unplug the power cord and remove the CMOS battery for few minutes).
After actually unplugging the computer from the power, keeping it unplugged for a minute or so and then plugging it back the system booted, and it seems to be working fine now. What that means? When I was having the HDD that failed installed in this computer all the troubles started with the same kind of behaviour - one day the system didn't boot unless I unplugged it from the power outlet...)