I'm assuming that you have checked all the outputs from the power source and they are delivering the correct voltage - otherwise the entire system would be completely frozen. In that case it appears that the chipset of the motherboard has failed. One simple test to confirm that: take the motherboard and the power source out of the case , make sure that no disk is left plugged in it, make sure that the only card left is the video one (ideally you should use a different one), take out the battery for the BIOS for 1 hour to fully reset it then connect the power and try to start it.
If the board is still non-responsive - the BIOS error message requesting to be redone doesn't appear - then the board is really dead. You will have to either get another one or buy a whole new system: motherboard, processor, memory, and video card. Or a complete new pc.
If the BIOS appears and it's accessible then the hard disk has died and it will have to be replaced - the problem was caused