There could be many reasons for this happening, but the first question is “what has changed?”
Did you do anything to the lawn mower such as change the rear tires? Did you pull out the tow rod to move the mower and forgot to push it back in?
Those are a couple of the common problems that we normally see in the shop and they are easy to sort out. When the back tires are changed, some riding lawn mowers such as John Deere what have wheel keys that fall out when you pull the wheel off. You don’t notice until you put the wheel back on and the transmission shaft just spins but the wheels do nothing.
The other thing that happens is the drive belt becomes so loose or it will break. There may be one of two belts depending on the make and model of riding lawn mower. Check those to see what condition they are in.
A linkage could also have broken but this is not very common.
Lastly, the transmission could be low on fluid or it could be broken and needs replaced. The newer plastic transmissions have a lever that can get stuck when a twig or rock gets behind it, so check that as well.
If you need professional riding lawn mower repair service, visit our Locations page for a lawn mower shop near you.