As you've said, fuse is UNlikely.... not just one light would go out, all backlights are connected to the same pin on the back, so if one goes out that's an internal thing in the speedo.
To find out what's gone wrong you'll have to take it out (4 to 6 screws) and open it (just some latches in case of Motometer and one small torx screw on the inside). AFAIK the two displays at the bottom are backlight by integrated LEDs, at least by the looks of it on my Motometer version, VDO and whatever the third one was might be different, so they could have a bulb there.
Most likely its a broken solder point, in which case it just needs some soldering, or the LED itself, that would be a bit more tricky but if you are fairly skilled you can remove the backlight led strip and check conrad electronics for a suitable replacement.
