Yesterday my son was mowing the lawn and when he was finished, instead of turning off the key, he just sttod up to get off. Of course the safety switch immediatly shut off the engine. He didn't tell me this and later in the day when I went to move the mower back into the storage shed it wouldn't start. I figured that since my son left the key on it discharged the battery. I have tried to recharge the battery and still the engine will not turn over. What could be wrong?