2007 craftsman lt2000 with Briggs and Stratton 20hp signal cylinder won't crank

Hi I have a 2007 craftsman lt2000 with a Briggs and Stratton intek 20hp single cylinder engine and it won't crank. Here is the story I was mowing and the motor recently had a oil and oil filter change so I drained the old oil and filled it back up with new oil and it started up fine and ran good. so I went out to mow and I engaged the blades and the motor sounded like it had no power so I disengaged the blades and the motor ran normally. I engaged the blades again and the motor sounded like it had no power at all and the motor eventually quite altogether and is locked up tight were you can spin it over with your hand. I have removed the spark plug and it wouldn't turn over and I've also checked all the safety switches and that didn't make a difference at all. I'm not for sure if I throwed a rod but I haven't seen any holes in the motor so what do you guys think happened to this motor. I've never had a problem with a Briggs and Stratton engine before. If anyone here can help me I would appreciate it thanks.