Small Engines (Lawn Mowers, etc.): Sears LT2000, engine crankshaft, lt2000

Question
I have the same problem listed. It just happened today and
I'm trying to find out the problem.
I put the shift into gear and it doesn't move. It happened suddenly. Stopped the unit to clean it, started it later and it would not go no matter what gear.

Answer
How old is the mower?  Please send me the mower model number, xxx.xxxxxx, so I can look up the transmission model.

Is the belt still on the pulleys?  Is the engine crankshaft bolt tight?  Are the transaxle mounting bolts tight?  Is the transmission belt in good shape?
Let me know the model number.
Eric