this problem is very very tough to diag, it could be any number of things really. Most common and easiest to try is PSU, your PSU might have failed and now its putting out wonky voltages and the system board wont POST with it. Could have a wierd stick of RAM in it, or the CPU or video could be gone or need reseated... if you have an asus board (and a few other models) try putting headphones in the jack on the board (if built in sound) it may tell you whats happening