The connecting rod has siezed to the crankshaft due to lack of oil, which then broke the rod. After tipping the mower up, the broken pieces have fallen out of the way, and now you have a flywheel that is connected to the crankshaft with nothing on the inside of the engine to provide resistance. so the flywheel will spin like a top. Most likely if you would look at the engine from the head end the block will be cracked on the left side. or if electric start behind the starter.