computer cannot boot up for no apparent reason?

[abigfanofnfs]

one day, while i was playing fsx on my comp, the computer suddenly restarted and when i turned on the comp again, the power light is on but no beep, no loading screen.
specs:
msi p45 neo fr
wd black 1tb
ocz 4gb dual channel
giga 1gb 9500 gt nvidia
what's the prob?