This is some sort of problem I created while trying to fix an initial problem. So here's what I started out to do.
It started when I was trying to fix a CPU overheating problem. I disconnected everything except the basics (CPU, 1 stick of memory, monitor, keyboard) so that I could figure out what was going on. Turns out the heat sink wasn't touching the processor completely and the power supply was a charred mess of exploded capacitors. So I seated the heat sink properly, replaced the power supply with a new one, booted it up and it worked fine. So I connected the hard drive and my video card, got into Windows, everything worked well, so I decided to just put it all back together.
That's where my problems started. Once I reassembled it, pressed the power button, all the lights and fans started going and the HD started spinning. Except the monitor didn't get any signal at all. Not even a flicker. Even weirder was the Num Lock key. It had a light, but it didn't toggle on and off normally. I'd press it and it would do one of a few different things. Either the light would stay on only while I was holding the key down, the light would toggle on or off, or the light (while already on) would just flicker a little bit. Caps lock and Scroll lock didn't respond to anything.
This motherboard is an Intel D915GAG. So here's what I've tried.
First I removed everything except what I thought the computer needed for me to get a glimpse of BIOS. Then:
Tried an LCD and a CRT, both of which work on another computer. Both are VGA. I tried them both at the same time, one on the video card and one on the motherboard VGA port. Nothing from either one. No splash screen, no BIOS, no Windows.
Then I removed the video card and tried both monitors on the motherboard's VGA port with no result.
At this point I was worried it was the motherboard, so I removed all the memory. It gave me the beep code for no memory. Also, to turn off the system, you have to hold the power button down for several seconds. I had to replace a bad motherboard in another system once, that power button turned the system off immediately when you pressed it until I replaced the motherboard. So I think I've ruled out a bad motherboard.
I reseated the memory, tried it in different slots, cleaned it off and tried new memory.
I'm using that new power supply that worked when I first booted it. So that shouldn't be an issue.
I removed the battery from the motherboard and jumpered the battery contacts to clear CMOS.
This system has got me completely stumped. I can't think of any more reasons that my display wouldn't work. My bag of tricks is empty. More than empty, I tried all that stuff multiple times just to be sure.
Hm. Well, I haven't got another video card to try, wish I did. In the past, this particular system would always revert to the onboard video if I took out the video card. The fan on the video card spins and everything when it's connected and it's actually only a month old. I managed to boot with both onboard video and the video card in the moments before I restarted this system to find that there was no video output.