I always have this problem on my 10 years old Athlon XP... when it refused to boot, I had this ritual: open the case, remove some cards, put back, loosen and tighten some screws, and  put all back. It usually worked.

When it didn't, I opened the case, grabbed it by one side, dragged it (not gently, it's the key) around, and put it back. And it works.