May still be the motherboard, power supply issue would do something similar. Does the fan actually spin? If not, the CPU could have burned out.
Does the computer seem like it's turning on or at least trying to? Does the hard-drive spin up? All lights go on?
You can try to reset the BIOS to defaults, either move the jumper over and back or unplug the computer from power and remove the CMOS battery from the motherboard for about 10 seconds.