I haven't come across that before, I've had my display go all hieroglyphic (ish) when it doesn't get decent power (running off a generator) and I have to power cycle it to fix it.
One thing to check is the ribbon cable that goes from the PCB to the front panel, it can come out of it's socket on the PCB quite easily, although the display is fed by another cable which runs from the PCB, under the ribbon cable to the screen, worth checking that it's connected properly. Otherwise, it would seem to be either a problem in the screen or a problem with whatever feeds the screen on the PCB
. You can get replacement screens, MJMP had some but they are the same as the screens used in Quadverbs and come in a variety of background/text colours. (https://www.ebay.com/itm/112535130034?ViewItem=&item=112535130034).
There's some chat about screen change in the MP2 section (http://adadepot.com/index.php?topic=1824.0)