Follow all of the advice above. Make sure the blade is disengaged, mower is out of gear etc.
I had a similar problem this spring and it turned out that the gear on the starter was jammed up with the fly wheel. On my mower, I was able to take the guard off the top of the starter/flywheel area and gently knock the starter gear down to disengage. Has been working fine ever since, however, I have ordered and received a new starter gear in case this happens again.