If you have the powerswitch and reset switch connectors on the right pins in the control cluster, you should be able to start the machine when you plug the power supply in and turn its case back switch on.
The power supply should have a standby lead #9 violet which should be at 5 volts when the power supply is plugged in and on. If this lead is at 0 volts, there is no power to run the soft switch. Usually this indicates the power supply is dead.
Alternatively, an extra brass standoff may have been left in under the mainboard. If this is shorting under a nonexistent screwhole, the power supply detects it and refuses to operate.
Or you could just have a dead PSU...