I have an MTD Yard Machines 20 h.p. Lawn tractor with the Briggs & Stratton I/C engine. The engine model # is 461707 and the type is 0143 E1. My son was cutting grass yesterday and said that the jerk and hesitate a little and then started making a clicking sound so he turned the blades off and then turned the engine off. We've tried starting it since then but it won't crank. There is a quick click but nothing happens. I checked the oil level on the dipstick and it was very low but not dry or empty. I've charged the battery also because I was afraid that continually trying to start it had drained it. Still nothing. Today I tried to spin it by hand and it would not budge. This has me thinking that the engine is locked up somehow. I have no experience with that at all. Any ideas? I brought the mower brand new back in 2000 and have not had any engine problems in 8 years, although I've gone through several belts and a few tires.