The symptoms you describe are typical of a drive motor failure. But just to test, take the robot out of the water, remove the filter basket and put it up on blocks so the treads are off the ground. Turn the cleaner on and watch the unit. Make sure the impeller spins up (there's usually 3 distinct high pitched tones it makes for different speeds) and that the drive motor moves the treads forward and backwards. If it fails any of these self-test processes, then you'll need to remove the motor assembly and take it apart. You need to look for water incursion. You can disassemble the drive motor or the impeller motor to see if the carbon brushes are worn out. Replacing the carbon brushes might fix the problem. Also Inspect the printed circuit board carefully with a magnifying glass. Look for any burned out components, swollen capacitors, or scorched copper traces. If the PCB is damaged, it's likely dead.
Maytronics doesn't make these internal components available as spare parts. So you have to try to rebuild what is there or abandon the robot if you can't. It is NOT worth buying a new motor assembly outside of a warranty replacement. Maytronics charges $450 for them and that's pretty much half the price of a new robot. Just like a washing machine that has failed, sometimes repairing it is simply not worth it.
