So I can get to the correct service materials, what is the serial number off the back of the mower deck? It is in the form of "MXXX-1234567"
Is this a gear-type transmissions or hydrostatic?
First thing I'd check is the belt (if it has one) and make sure it's not jumped off the pulley on the transmission. To do this , first turn the fuel valve to OFF, make sure the fuel cap is on tight, and disconnect the spark plug.
Next, tip the mower over so the right side wheels are down and left side wheels up. Locate the belt. Depending on the model, the belt made have slight slop, no slop or appear to be loose/disconnected.
If you don't see a belt at all, you may have an older shaft-drive model. With that serial number, I can go farther.
Inspect the drive clutch lever and confirm it's connected at both ends and operating smoothly..