| If you're getting the LED on the motherboard you've got +5V, if you've got access to a multimeter check one of the 4 pin connectors going to a hard disk etc, should have +12V there also.
Modern PC's don't use a direct switch to the power supply, it comes off the m/board, so as has been said investigate your power switch first.
You've reseated your CPU, RAM and video card, these are the only things you need plugged in to get your PC working. can you remove the m/board from the case and try powering it up externally? when you knocked it over you could of moved the m/board slightly to be shorting out on the case etc | 
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