Unplug the power switch connector on the motherboard. unplug the power cord to the power supply. Plug it in again, does the computer boot? What motherboard is it?
Do you have another power supply somewhere? - You have at least two possible scenario, 1 motherboard is defective, that's why floppy drive doesn't work or 2) power supply is defective - but this does not explain why your floppy does not work.