If you have the unit open, then I'm sure you already checked that all the cables coming from the display, do connect to their destination as they should? (I wouldn't know where that is, without disassembling mine and looking for it).
Also, do I get this right, that you never saw the display working on this unit? If it DID work, and suddenly wouldn't, then what I'm describing below is NOT the problem.
The display looks completely out in the picture... but just to be sure, have you tried shining a flashlight into it, to see if it actually displays any characters (but very dark)?
I haven't seen this issue on a d8b yet, but old LCD screens usually illuminate the characters via a backlight "foil" (or "EL-foil", that is mounted behind the display. When a display gets old, that foil gets dimmer and dimmer until it finally doesn't light up anymore at all.
You might have seen this on old synths and other gear with LCD screens, where you first have to fiddle with the contrast to make it readable, but eventually it will become useless.
The good thing is, that this kind of EL-Foil is relatively cheap, and usually not that difficult to replace. I had to do this for a couple of my old pieces of gear already (e.g. Dynacord ADD-One), and the display becomes like new again. Another Mackie Control Universal (old version) I have is on it's last leg, very dim, and will need a replacement, soon.
Here's where I ordered the EL-Foil for my Dynacord Sampler at the time:
http://www.midi-rakete.de/eng/elfolie.htmI didn't look if they have something pre-made for the d8b, but they can make custom sized ones to order, if the backlight is the problem.
Again, I haven't seen this being a problem on a d8b, but could imagine that the display could go bad, if someone leaves the console on for years at a time
...anyway, check with a flashlight. If it displays characters, then it's probably the EL-Foil. If there are no characters visible, then it's something else (display itself, connectors, or any component controlling the display).
That's at least my guess...