Try turning the steering wheel and stepping on the brake. The steering may have had pressure on it and was jammed against the locking mechanism when you turned off the car and released the brakes,
If the gentlemans suggestion above mine did not work, there is the off chance that the problem either lies in the tumblers of the lockcylinder or your key is messed up. Fortunately the cylinder is only 16.99 at Auto Zone and the cylinder can be easily removed and replaced.
This just happened to me.. I have AAA and they sent a locksmith who fixed it in 30 min by replacing the tumblers.. So I would call a locksmith.. hope this helps