In many cases, it's simply a poorly seated ribbon cable that need to be removed and reinstalled into its socket connectors.
Some (many) manufacturers use "hot glue" to try and hold the ribbon cables in place. I never thought that was a very good idea as hot glue stops sticking after a short time when used like this.
Take an older radio apart and you will find the hot glue will just pull off and is not really holding anything any longer.
A better solution would have been to use a better connector!
The LCD displays also use a conductive rubber like material to transfer the display data from the logic boards display controller to the actual LCD module.
Sometimes there is not enough pressure between the LCD module and the circuit board so the conductive rubber strip stops connecting.
Sometimes this can be fixed by tightening screws between the two parts so they are held together tighther and sometimes removing the glass LCD module and the conductive rubber strip and then cleaning the contacts on the PC board ever so slightly with something like a pencil eraser is all it takes to make a good connection again.
If you tackle this one and it uses a ribbon cable in sockets, pay attention to the sockets. There are several types. Some just use pressure to insert and hold the cable while others use some form of release/retention mechanism that must be released before you pull the cable out and reseat it.
That retention mechanism is a mechanical plastic part and is easily broken.
If you are not familiar with this at all, it would be best to find someone that does electronics repair as once you break the retention mechanism, your display and scanner may never work again!