Should be 2 drive belts a long one and a short one under the seat .
Check that it is still connected
Other than that there is a spring that puts tension on the clutch pulleys
That spring has a habit of falling off, cutting through the mounts or just breaking
I usually stand the mower up on its end ( remove battery & seal off fuel cap first ) to check everything .
NAturally you need to remove the deck as well
When you push the go pedal it should pull the clutching pulleys to tighten the lower belt
There is an adjustment but generally it will be one of the springs.