Grass or debris is likely jammed around the drive shaft. It's a common clog.
Fix it:
- Remove the battery pack.
- Tip the mower safely. Inspect the drive shaft area (near the wheels) for wrapped grass, string, or leaves.
- Clear all debris by hand. Do not use water.
- Check that the self-propel control lever/bar is fully engaged when you press it.
If that didn't work:
- Internal drive mechanism failure. This requires a qualified service technician. Do not attempt to open the gear case yourself.
Heads up: Always remove the battery before checking for clogs. Never operate the mower without all safety shields and devices in place and working.