Have MTD Yardman 13.5HP 38" Shift-on-the-Go --- Transmission will not engage when I press in the clutch... The drive belt is in place... not broken... What steps do I take to determine what is wrong...

Hi,
The machine was probably turned off with clutch engaged.
To remedy this put in neutral, place Shift-On-The-Go lever in highest speed, crank engine up, release the clutch/brake pedal and wait to the whirring sound of the variable speed pulley. Once you hear that everything is back to normal.
Just remember to depress the clutch pedal prior to shutting down in the future.
Peace,
Clarke