lansard

I have a 2003 MTD garden tractor with a 25 hp B and S OHV  Intek engine...when I turned off the engine after a few hours use it made a large noise...never heard that noise ever before and it sort of startled me....this morning I went to start the mower and all it does is turn over...the spark is good and so is the compression....fuel is getting through judging by the smell and wettness on the spark plug....I am at a loss...does anyone have any ideas as to what is wrong...I thought maybe one of the safety features got hung up or something like that, but that wouldn't explain the large bang I heard when I shut off the motor....any advice would be appreciated.....