The circuit board is one of the components that can be damaged by events such as lightning. The robot will often display the error message "no loop signal," even though the LED shows a steady green light. In this case, try disconnecting one of the boundary wires; when you do this, the LED should start blinking blue. If it doesn’t, the circuit board is likely faulty.
Don’t forget to pair the robotic lawnmower after replacing the circuit board. You can do this via the menu -> security -> advanced -> new loop signal.