You're likely going to have to open it up to find out, but I know from working in some other advantest instruments that they tend to use fairly standard LCD modules, if you actually need a replacement, they should be available. Similarly, not many service manuals, if there is something, it's likely on KO4BB... and I only see a product brochure.
First open it up and make sure your cable isn't just loose at either end. Then give it a good jiggle and see if you can replicate the issue with the same sort of thing. Then figure out if it's a cable or connector thing which is likely easy to fix, or if it has to do with the cables built onto the LCD glass, which would probably need a replacement if you can't use heat to reseat the attachment or similar.