umm, yes it could be, take the graphics card out and use the default onbored one , i hope that's what you have, make sure you take the card out.. graphics card requires a large partion of the watt.. most of the watt is vis graphics card
mine uses alot.. it requires 550 mine(not the whole card, with the card and the pc components) and I had a 450 before that and it eventually clocked off.. the lights turned on still but the PSU just didn't turn my PC on..
Have you tried taking it alll to bits.. bit by bit.. and put the mobo on the metal case of your pc.. try that.. that's what I had to do to check mine, take out 1 memory card also just to make sure it isn't a memory shorting the power incase it blew a memory slot.
Give it a shot, you might solve it
Tip: Fuse for the plug, test the moniters plug to your pc as mostly their the same .. try that also , could be the fuse
Give that a go and let me know how it goes, I'm at work, so I'll check back at the office computer on my brake in 2hours , 4oclock gmt
so if you still haven't I'll think of other things until we draw our attention that it's a problem with some hardware, lets not give up to easy
I almost gave up on mine.. spend 30pound on a new PSU and still wouldn't turn on.. eventually after 30mins I figured it was a faulty memory slot.. which I fixed somehow.. lol and now it works