Refer to this article if Laser power LCD display not lit>>.

If the laser power LCD display lits up normally, check whether the "water protection" on the  LCD reads "on".

If it reads "off", 

• Check whether the cover is  closed
• Check whether the debris tray is fully inserted.
• Check whether the coolant flow is normal.

If all these have been proved normal, 

1. Short-circuit the cover interlock and debris tray interlock.
2. Check the "water protection" on the  LCD.

If it reads "on", then the interlocks are faulty. Replace them with new, identical ones,

If the problem persists, short-circuit this port on the laser power supply (as shown).

Check the "water protection" on the  LCD.

• If it now reads "on", replace the sensor of the water protection. 
• If it still reads "off", replace the laser power supply.