Your monitor could be the problem. Is it an old CRT one? I remember a monitor that used to do pink tones and stuff because it was on the way out (or maybe it was my TV, I can't remember).
It could be your graphics card. I used to have an NVIDIA 7900 (I think) that stopped working because dust clogged up it's fan causing it to overheat (although many people seemed to have problems with it too).
Maybe you just knocked a cable out. It took me a few minutes earlier to turn my PC on after I knocked the kettle lead just enough so I couldn't tell it was out. Try wiggling them or something like the other guy said.