Look for a clogged air filter or possibly a choke that is not pulling off to start. Is this a situation that this is the mower's maiden voyage for the year?

With engine running (have a helper hold the bail or tie it down) touch the governor lever on the LH side of the block at the rear. Pull it back to rev the engine. If it revs, you know the problem is in the throttle control. Is this a new mower? You might have to return it for repair if you can't troubleshoot it. If it's not new, the assumption is it ran okay last season and something has happened in the interim.