A photo of the light might help, too. I was assuming that the light in question was a fluorescent fixture like the one in my 1983 Excella. Broken filaments like dwightdi suggested would apply to one with incandescent bulbs.
The ThinLite fixtures in my Excella are different from the usual fixtures that have a couple of commonly available 12" tubes. They require an unusual size tube that is more difficult to find. My experience with these fixtures is that they are unusually unreliable. All ThinLite products seem to have short ballast and tube life, but especially those in the light fixtures that also serve as vent covers.