Could be power supply problem, could be some debris such as hair wrapped around the pump impeller or it could be a bad drive motor.
Test the power supply without the robot plugged in. Insert the probes of a multimeter set on A/C 200vac. Power should be 60 watts +.
If the power supply is good then check the pump impeller by removing the screen on top. If clean then there is an internal problem with the motor and/or pump and you will have to take it to a repair shop.