These units are now very old, so there is a limit to the support we can provide.
That said, your problem may have one of several possible causes:
1) The display brightness may be set low. If this is the case, you should be able to control it – with the right light on the display, you can still see what it is trying to say.
2) The display itself may be faulty. If this is the case, we can't help (they don't make them any more and we are long out of stock). Look at previous threads for a few eBay suggestions.
3) The circuit on the motherboard that generates the high voltage (1000V) for the backlight may be broken. This is repairable.