Won't turn on

My computer wont turn on. D:
I press the button, nothing happens. Sometimes, it lights up, but shuts down again immediatly. Is it the powersupply?

HLEP ME D:

1. the power suply is fucked, this is the most likely. i used to have this happen ALOT with $15 power suplies in the pentium II days
2. mobo is fucked
3. try taking all of the pci cards out, and unplugging every cable besides power to the motherboard, then try it, does it turn on?

if 3 works, do the old trial and error method of plugging stuff back in one at a time, then you know whats shortint out