This problem struck me yesterday and I have experienced a similar problem before, but I suspect it is the PSU that's a little out of date this time. I'm on a different computor now, but when I start the other one nothing shows up on the screen, at all, it just stays black - I know the computor is working and that, I can hear the sound of my anti-virus program for example, but the screen wont go on, and restarting it only causes it to get a little brighter and then say "Monitor going to sleep", then going black again.
Last time I had this problem I solved it by buying a new video card, but that time the screen did work and I could enter safe mode, it just went to sleep after being on for a few minutes.
Anyway this time I suspect that the PSU has gone poof, since when I bought the video card I'm using now my normal power supply didn't have enough juice so I had to plug in some other toy I got with the card to give it some extra power, but I'm guessing that now it's kinda cooked. It's nothing wrong with the screen, I have tried replacing the screen I normally use with another one with no luck, also I've tried using the normal screen on a different computor (this one I'm typing on now) and it works just fine.

Not sure how pimped out my PSU is, don't know anything about it and can't check now but my video card is named Asus EN8800GT G
(512 MB) and afaik this card isn't THAT old and should work so I dunno. Hope this is enough info.

Obs. As I said this happened yesterday, it first went black around 15 pm and then after unsuccessfully trying to fix it I went to the TV and returned around 20 pm and it did work. I used it til around 00-01 am, shut it down and then next morning that being today it's retarded again. If this erased some possibilities.

I have a similar problem, i recently ordered an EVGA Nvidia Geforce 260 GTX and i didnt have enough 4-pins to adapt to the 2 4-pin to 6-pin adapters needed to plug in the card. So went and got a couple 4-pin Y splitters and now i have enough to at least plug my video card in, but when i did plug it in, my monitor didn't display anything. I thought something mustn't be plugged in right and i tinkered with everything possible. I had already turned off the on-board graphics card in my motherboard and switched the BIOS to pci-e graphics detection (the only option other than Onboard) so that my previous video card would work. But it gets worse... After many restarts and trying different options with my onboard video, my onboard video stopped working after a restart. Which is where I thought it's gotta be my power supply. I tried my old video card and that didn't work either. Thing is, all of my fans and everything are running fine with my power supply but i think iv'e fried it with the adapters to this huge new video card. Its a 2006 power supply but it has a 600W Max, and 500W minimum required for my card, i thought i could get away with splitting a couple of 4-pins. Also my power supply's fan is operating in spurts, wich i never noticed before and im thinking that's a bit off. I'm having my slightly more computer-smart brother come over tomorrow and see if my power supply theory is correct.