DIAGNOSIS This is a drive motor failure. One or both of the drive wheels cannot engage. The wheel module gears are likely stripped.
FIX Tip the mower on its side (power off). Manually try to spin each drive wheel. The wheel with stripped gears will spin freely with almost no resistance. You must replace the entire drive module (left or right) for that wheel.
IF THAT DIDN'T WORK If both wheels have resistance but the mower still won't move, the issue could be the mainboard drive circuit. This is less common than a physical gear failure.
HEADS UP Continuing to run it with a stripped drive module will destroy the plastic gear housing. Replace the module as soon as you confirm free-spinning.