Could be a sticky power switch.
However, every 6 months or so my PC will not start. I have tried everything but the only way I can get it to start is to unscrew the PSU and move it about an inch. Switch back on and all is fine for another 6 months. No, moving the cables only or opening the PSU and fidling with the innards doesn't make it work, only unscrewing it does the trick, screwy.