Can't get a display on my monitor with either DVI or HDMI

PC's been working fine for awhile, but I had to unplug my DVI cable, and now I can't get an image on my monitor when I put it back in. Nothing else has been unplugged. I tried my HDMI as well, and that didn't work. I got an image once on my TV via HDMI, but I unplugged it to try the DVI again on my monitor and nothing. Haven't been able to get an image on my TV again either. Any ideas what might be my problem?

I think the problem with my DVI is I accidently grabbed the wrong cable (I was moving my computer to my college apartment). I grabbed an 18-pin cable which may have been lying nearby, and I think my monitor only supports 24-pin. Reading up on my monitor, 24 is what it came with so I somehow mixed those up. Will test this out when I buy a new cable...

Still not sure why HDMI won't work.