|When the power supply died it may have damaged the motherboard. |
PS/2 keyboard and mouse are not hot swappable. Hopefully you did NOT connect either while under power. That could damage the PS/2 controller. Most older motherboards have a setting in the BIOS that must be enabled to be able to use USB hardware in the screens prior to entering Windows.
Did you connect the secondary square, 4 pin power connector to the motherboard?
Only thing I can think to try is to unplug the computer and press the start button a few times. Plug the computer back in and try booting again.