The self-propelled mechanism on a lawn mower operates through a drive pulley, a drive belt and front pulley, connecting the crankshaft to the front wheels. When this system stops moving the lawn mower forward, a loss in the transfer of power from the engine to the wheels is occurring somewhere in the drive system.
Before attempting to service any area on the lawn mower, disengage the ignition switch and wait for any moving parts to stop. You'll want to park the mower on a flat area of ground, and wait for it to cool off for at least 15 minutes. Unhook the black rubber boot connected to the end of the spark plug, and set this wire onto a metal point on the engine block to ground it. Always wear proper work gloves when working on or near the mower blades and drive system.
Accessing the Belt
On most self-propelled lawn mowers, the drive system sits directly underneath the engine, which allows the drive pulley to connect directly to the crankshaft, so when it spins, the whole drive system moves in sync with it. A handle, usually on the lawn mower's handle, allows the operator to manually tighten and loosen a spring mechanism. When the spring mechanism is engaged, the belt tightens on the pulley and the lawn mower drives. Verify that the lever is disengaged before removing the drive belt.
Removing the Belt
Reach underneath the engine to the rear drive pulley, and connect a socket wrench to the center bolt in the pulley. Loosen the nut slightly, and pull the drive belt off the rear drive pulley. With gloved hands, roll the pulley while tugging the belt slowly off the rear drive pulley and the belt guides. Slide the belt off the front pulley and out of the engine, and see that no debris or objects have fallen underneath the engine and pulleys.
Fixing the Problem
Inspect the condition of the drive belt. If the belt looks frayed, torn or stretched out, replace it with a new drive belt. Spin both pulleys with your hand, verifying that they spin freely. If they can't spin freely, allow a service professional to disassemble and remove the pulleys and replace them. Refit the drive belt back to the mower in the same way it was removed. If necessary, draw a diagram of the belt's routing to ensure proper reassembly.