The obvious answer is that the bulb has blown!
If it is an LCD display then they use a kind of fluorescent backlight and the electronics driving that can also pop.
If you are lucky and it uses a filament bulb - not likely on a digital display more common on an analogue pointer dial - then it might be replaceable fairly easily if you can take the beast apart.
An LCD digital display might need the whole display module changing. And some I have seen are soldered onto the PCB others are socketed (hope you have the socketed variety!

).
But what could be worse is if the driving electronics have popped. Again they could be integrated into the display module. But just as equally they could be on the main PCB of the unit. Which would be a b***er to change. In fact it would probably be cheaper to bin the thing and buy a new one.