Why doesn't the display work?

If the display went from good clarity to no clarity, its not the display itself. Check the Dimm chip in the Formattor, or change out the Formattor. You can also do an engine test which will tell if your DC Controller board is okay. Sometimes known as a line test, most laser printer will have a tiny hole on the side of the printer that can be pushed in with a pencils tip.

Your low voltage power supply might be bad - Let me know if you need a part number, HP laser printer parts can be found on line.