[villagetinker]

OP, this is usually a simple fix, one of the tubes is not installed correctly, or was not turned far enough (or too far) to make connection on all of the pins. Try turning the tubes slightly while the light is on. If this does not work, then you will probably need to replace the fixture. You might want to consider an LED based light, same size as the fluorescent, lower electricity use. Try Lowes, home depot, rural king, etc.
Also, a word of caution, I have helped 2 neighbors with problems with these lamps, and both times it was loose mounting screws, the installers did NOT bother to locate studs, and just screwed into the drywall!!! In both cases the fixtures came loose, one almost fell down. So, if you replace, make sure the new mounting goes into studs, or suitable drywall mounting adapters. Send me a PM if you want to discuss further or talk to me. I would offer to help, but we have company for the next 2 weeks.