(Figured I'd combine my comments into an answer for anyone's future reference, since OP's been MIA since '12)
When diagnosing an engine that won't start, it's helpful to remember what all is needed in a working engine: fire. Since a non-starting engine won't have "fire", a good checklist to run through is:
Use PB Blaster(the BEST),NOT WD-40(Water Displacement Formulation 40) or any brand of penetrating solvent,Liquid Wrench,etc. An impact wrench is a formula for disaster. Use a breaker bar to get the plug to move just a bit is good,apply more penetrant and,hard as it may to do so,WAIT. Days,even.If you can get the plug to move, even a bit counter-clockwise,re ...
In a word: NO!
Please do not use an impact wrench on these. You run the distinct chance of stripping out all of the threads in the head, which will cause a huge nightmare for you having to have threads replaced (Heli-coil or the like). Just use a socket/ratchet and you'll be fine.
I am pretty sure the spark plug is broken but I am not sure
That's almost certainly the issue. Replace the spark plug and try again, if the problem is fixed, you're all set. If not, then there are more things to check:
The ignition coil, on this vehicle I believe it's located above the spark plug. Try swapping two of them and see if the problem follows.