- is the powerplug connected to both wall outlet and PC?
- is the power button connected to the mainboard? (friend of mine forgot it once )
- does the inside of the pc smell like burnt material?
- is the inside of the pc pitch black in some places where it shouldn't be that way?
my guess is a broken part, don't know which one.
Are there any diagnostic lights on the mainboard? They helped me out once.