Yeah, this one's common. Either the drive cable came loose or the self-propelled unit's motor failed.
First, check the connecting rod assembly (part number 2825245001) on the handle. See if it's still attached to the switch actuator. If it's disconnected, the drive signal isn't reaching the unit.
If that looks good, the issue is in the self-propelled unit itself. The motor & gear assembly (part number 2826577002) can burn out, especially in very thick grass. You'll need to replace the whole unit.
If that didn't work, inspect the internal wire assembly (part number 2826843001) for a break. Also, the switch electric assembly (part number 2830923001) could be faulty.
Heads up: Don't force the mower through super thick, wet grass. Use a higher height setting to reduce strain on the drive.