Replace the tube.

A clue will be to look at the tube closely. Grey or black bands at either end indicate a fluorescent lamp at the end of its life cycle. Replacement of the tube is the next logical step.

If that fails, try remounting several times as positioning can be tricky, you get into checking the ballast and tombstones and it slides into being cheaper, and more time efficient, to simply replace the entire fixture. A thirty year old fixture is typically ripe for replacement.