Monitor Won't Connect to DVI or Analogue

This problem is really perplexing me as everything has been working fine until just now.
Basically I've been upgrading various aspects of my PC and recently I got a new graphics card, a Radeon HD 66870 and everything was working for great for the first day running it. Graphics were so much better and slicker but I set my PC to sleep when I left the house and now my monitor will not connect to my PC.
Here's what I have tried:
-I've unplugged my DVI cable on both ends and plugged back in.
-Switched off the PC and monitor and restarted.
-Taken out my new graphics card and put in my old Nvidia 9400 GT which doesn't use DVI, but still the same problem.
-However weirdly enough, when I plugged the monitor into my laptop as a cloned screen, my monitor displayed my laptop screen perfectly.

I'm really stumped on how to get my PC and monitor to connect and I don't have an alternative monitor to use, so I now can't use my PC.
Please help.