This could be a multitude of problems. I'd do the age old T&E and eliminate problems.
1. Check the signal cable from PC to Monitor
2. Check the monitor on another computer if possible
3. Check that the video card didn't become unseated in the expansion slot
I've had VGA and DVI cables both fail on me in my years of being behind the keyboard. Sometimes that tight turn can just take a toll on the wire and cause it to fail. I've also had video cards die out. I've even had power supplies, motherboards, processors, RAM, you name it fail on me. So it could me a multitude of different causes that has given you this issue. Remember, start with the basics and work your way up.