DIAGNOSIS The robot's software has locked up. A hard reset is the equivalent of pulling the plug on a computer.
FIX Flip the robot over. On the side, you'll find the power switch. Turn it to the OFF position. Wait a full 10 seconds. Turn it back to ON. Place the robot on its charging contacts in the base. You should see the light ring illuminate. This clears the temporary error memory.
IF THAT DIDN'T WORK If it's still frozen, try a factory reset. With the robot ON and off the charger, press and hold the DOCK and MAX buttons simultaneously for about 10 seconds until you hear a tone and the lights cycle. This wipes its Wi-Fi and map data, so you'll have to set it up again in the app.
HEADS UP The factory reset is a nuclear option. It will delete your smart map, so the robot will have to re-learn your entire floor plan from scratch on its next run.