My computer has been working great for several months, when one day I noticed the fans/HDs were running but the display signal would not return when I wiggled the mouse (optical; there was no red light underneath); the computer had been left running beforehand. Anyway, I MANUALLY reset it and it booted back up just fine. A week later however, the computer did the exact same thing, and now it acts like it is booting up (fans etc. start) but there is no display, every time I try. Also there is a fancy blue light that shines out the front of the case that also won't light up anymore.

I ruled out the monitor (using another computer). I pulled out my agp video card and tried the onboard video. No luck. I tried to reset CMOS by pulling out the battery for a few minutes. nothing. I disconnected and RECONNECTED everything. Nothing. Then I diconnected everything except psu, cpu, ram. And after than I tried a different cpu in the motherboard, and it still won't boot up . .

The only thing I can think of would be either the motherboard or the psu. Any suggestions?well you did a good job troubleshootin give yourself a pat on the back. good job
Thats everything I would have recommended.
BUT one las thing.
My aunt had this same problem her machine would boot but no display. i fix it in 2 minutes.

Turn your brightness up o your monitor and the reast of the stuff for you display to.
This is pretty much what I can think of since you did everthing elsehey subfusca,

when u said u tested the mothboard monitor jack, did u still have ur agp video card removed? because when some hardware is simply removed, and not uninstalled, it might give u a plain black screen. im saying this because, when i got rid of dial up, i simply yanked out my modem, and my monitor went black. i had to actually uninstall the modem first before removing it. with this being said, u might have caused a problem trying to solve the initial problem. u might WANT to put back ALL of ur hardware, even the video card, and then test the motherboard monitor jack. then if that works and ur video card doesnt, u KNOW ur video card is fried. unless uve done all this and i just wasted 10 minutes of my time. thanx guys. i actually tried the onboard video before i pulled the vid. card--which works fine when i switch it to another computer. :-?Quote

Turn your brightness up o your monitor and the reast of the stuff for you display to. This is pretty much what I can think of since you did everthing else