If for an reason the 24V and 0V are applied directly to the LED backlight the backlight array of LED will be damaged.
Perhaps buying a new HMI to replace the existing one and send the damaged one back to us for repair may be the quickest solution.
To replace the LCD, you need to remove the LCD module from the HMI by desoldering the 16 header pin and replace it with a new LCD.
Alternatively, if you disassemble the LCD module, you can retrieve the light box unit in the LCD which is an independent unit that can be taken out of the LCD module assembly and replace with a new one. However, you will need to align the contact between the glass and the PCB properly otherwise there will be display problem (such as missing segment).