Ok, I think I pretty much have an idea of whats wrong, just want to see if others can confirm.
About two weeks back I put together a pc for my mom. Long story short, she had a Dell computer that got fried in a storm and I tested and harvested what I could, pretty much just replaced motherboard with one I had in another pc for about 2 months and was able to use the rest.
Worked fine for a couple of weeks, not exactly heavily used though, and last night apparently it crashed with a blue screen after being used for the better part of the day. Now whenever it is turned on the monitor shows no input signal. I thought it might be the video card but after swapping in one from a different pc that I know works I still get the same thing. So I tested the monitor on another rig and the monitor works fine.
At this point I'm thinking the CPU is bad. The motherboard was rock solid in the other pc for 2 months with pretty steady use, so I don't think it could be the pci express slot (at least I hope not). It shouldn't be the ram either as it was also used and stress tested in other pc's for quite a while after the Dell was fried.
The thing that throws me off is that after swapping out the video card the series of beeps I got on boot up was indeed for monitor or graphics card error (which was what lead me to test the monitor). After testing the monitor on another pc and taking it back I no longer get any kind of post beeps. I just don't see any way that the motherboard or pci e slot could've been damaged, unless someone else could enlighten me, so I'm thinking it's the CPU, just want to get a diagnosis from a few others possibly before I shell out any cash for a new one.
A bad CPU should give some kind of beep code. Does the computer seem like it's booting up properly even without the video signal?
Is the monitor connected through DVI or VGA? If it's DVI, try connecting it through VGA instead. You could also try unplugging the power on the monitor for at least 10 minutes to reset its device ID. Sometimes the DID gets corrupted somehow and sends bad information about the monitor to the video card, which causes the video card to send a default VGA signal instead of DVI. If that's happening the monitor would work fine through a VGA connection, but not at all through DVI.
If that doesn't work, and you don't get any beep codes on startup, then I think we're looking at either a corrupted BIOS (remove motherboard battery for 1 minute), a faulty power supply, a broken motherboard or possibly a defective VGA cable if you used a different one when you tested the monitor on the second machine. The only way to narrow it down is to keep swapping parts until you find the one causing the problem. With no beep codes though my money is on the motherboard, the PCI-E slot in my old one just gave up on me one day without warning so who knows.