The height adjusting lever is likely jammed with grass clippings or debris.
You'll need to free it up. First, make sure the mower is off and the battery is removed.
- Lift the mower so the wheels are off the ground. This relieves pressure on the mechanism.
- Locate the height adjusting lever (part number 32). Wiggle it back and forth while trying to move it to a different setting.
- If it's still stuck, look where the lever connects to the wheel assemblies. Clean out any packed grass or dirt around the axle and the depth stop plate (part number 34).
- Check the front axle set (part number 38) and the wheel assemblies for free movement. A bent axle can bind the lever.
If that didn't work, the lever mechanism itself may be damaged internally. You'd need to inspect the torsion spring (part number 19) and related parts under the deck.
Heads up: Regularly clean the height adjustment areas after mowing, especially in damp conditions, to prevent this.