At a loss, computer/monitor has no signal.

[Helltech]

First off I didn’t know if this should be in dead machines, or monitors. This problem doesn’t really fit in either, so I’ll post in both and if a mod wants to lock one because its not in the right area I understand.
Ok, so this isn’t a big deal as I’m just trying to put together a little working computer for when I have guests, something they can use to check mail, or whatever. I have a lot of old machines, three emachines, and a very old custom computer. So I figured I could put something together and get it working, and the fact is I did have a working computer, and this is what happened. I turned it off, and when I went to turn it back on (I didn’t change any configuration) I hear an electric pop, so I’m sure something got ruined but I have no idea what it could be. Everything “seems” to work. But I don’t have any picture on my monitor.
It’s not the PSU; I’ve tried three different ones. It’s not some problem with interference from input devices, as I tried to turn it on with no input devices (mouse/keyboard). I’ve tried two different motherboards, still no signal; I thought that was really odd…. Unplugged the CD-drive, I don’t know how that could interfere but I thought I would try. It’s not the monitor because I’ve tried two different monitors and both of them work with my main computer. It’s not the monitor cord I’ve tried multiple ones. I tried a graphics card in both motherboards, nothing. Tried no graphics card, still nothing.
The only things that I haven’t, or cant test are the ram, and the hard drive, I doubt it’s the hard drive as one; I don’t see how that could cause a problem with the monitor, and two the “pop” I heard came from the motherboard, I think.
So could the ram be causing problems with no video signal? I don’t think so, but I don’t know what else it could be. I honestly don’t know what to try.