The lawn mower will not go forward or reverse, What should I check to fix this problem?

Is the tow valve rod engaged? Have you checked the drive belt? It could be broken or off the pulley. Also it will not go forward or reverse if the park brake is engaged. Have you had a rear wheel of and not put the key back in?

Good answers there esquire1, I agree, a good place to start. But one question, what if any were the symptoms to the failure?

Agreed good start The tow rod is used to manually push most mowers with a hydrostatic transmission (you just push a pedal/lever and it goes faster with more pressure) The tow rod is usually in the back of the mower but sometimes found underneath but always as part of the transmission. Check to make sure all the belts are in place and tight (with the clutch/break not engaged). Ensure each belt is in a route with the pulley and not riding against metal hangers ( they most likely are in place because you would normally smell burning rubber if they were rubbing) Good luck!

Also check these thing with the mower NOT running wouldn't want yah to lose a finger :)