It is usually because the motor block unit in the robot has become flooded with water and shorted out the internal controller/driver board.
The power/controller unit does two things, it provides power to the robot via two wires, and communicates to the robots controller/driver board over a single wire. When the power/controller turns on it sends voltage to the robot and attempts to communicate with the robot. If no signal is returned to the power/controller the display shows error 10 and then the power/controller shuts down the power. This happens quickly (a second or so).
Chances are that if your unit shows error 10, the power supply is working properly. Now, the cable or the connection to the robot can be faulty, but it's pretty easy to disconnect the cable and check for continuity on all the wires (four wires in the cable- but only 3 are used).
Feel free to PM me if you have any other questions.