Its where the paperweight went after hitting the HD.
I think you have guessed rightly - mother board is most likely to have suffered.
This would be a reason why the on/off switch is not working. It merely activates an electronic relay to wake up the PSU.
You must give it a chance. Disconnect power supply from all data drives HD and CD etc (after checking the PSU to motherboard cable is still connected and the RAM is still home in its socket/s).
Switch on and if it lights up, it will go as far as trying to load windows before giving an error message.
If not it is time to think of a new motherboard and all that entails