This happened to me once. I had a bad starter a while back, when my mechanic installed the new one they knocked loose a ground wire..That ground wire controled my radiator fans so the car was overheating. Thats when the shifter became locked. Well I took the car to another shop and by re-grounding the wire the shifter worked fine again. Sooo most likely its a loose wire or some sh!t. Could be something else tho