Most likely the Monitor volume components have failed.
The easiest thing to do, is to borrow another monitor from a friend or buy one (They are about $100 on TigerDirect) and plug it in and see if it works; if it does, you're old monitor failed, and stick that one in a closet for emergencies and use the new one, or if you borrowed one to test your machine; buy a replacement!!
If this doesn't work, you are looking at very complex troubleshooting involving testing your Hardware as mentioned, and backing up all your data to external media and performing a W7 reset or W7 reinstall from Factory Image. If you get to this point and test your internal PC hardware and all passes; this is your only option unless you take your PC and...