It could be anything.
You are going to have to physically look inside the tower to see if anything is loose.
A good place to start is the RAM. One or more sticks of RAM could have fallen out 
or come loose. Re-seat them and try using the system.
Another problem could be the heatsink/fan on the processor.
Look to see if the fan was knocked off or the heatsink had jarred loose.
Yet another is the hard drive. The mishap could have permanently damaged the internals 
of the hard drive.
The list can go on.
As I said, the mishap could have damaged anything.

Good luck!

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