Hello, my name is***** will offer some information that may help you diagnose this problem.
First, I am assuming that when you say it will not turn over you mean that the engine is locked up and when you try to start it nothing turns, not the blades or the belt that drives them or the pulleys that the belts drive...not even the starter. If this is not correct, please let me know.
If you haven't done so already, check under the mower deck thoroughly for something, perhaps a piece of the stick that you hit that is wedged between the mower deck and blade. Check to see that the blades will turn freely with the PTO off or the belt tension device disengaged.
Check to see that the drive belt has not been un-tracked and wedged between a guide and a pulley.
With the transmission (if manual) in neutral, see if the mower can be pushed and the drive wheels turn.
If possible, take the starter off and see if it works properly (ground the starter and turn on the key) and do not get in the way of the working end of the thing.
With the starter off, see if you can turn the engine over by turning the pulley attached to the engine.
This is a process that should either identify common problems for an easy fix or indicate that something might be broken inside the engine.
If these steps seem inappropriate for your mower, please send me info on the model number of the mower that you have and I will adjust the answer to fit.
I hope that this helps you identify the problem and it is one of the easier fixes. If it is a problem internal to the engine, you may face choices as to whether a new engine is necessary or not.
I am interested in whether this helps or not. Please inform me of what you find and it may guide me for additional steps.