There are a couple of reasons they quit:
The covering over the lamp gets too dirty. (Take it apart and clean according to manufacturer's recommendations)
The lamp burns out (replace). You cannot tell by looking at it. Don't look at it when it is on! UV bulbs can hurt your eyes. Some units include an indicator of some sort to let you know that the bulb has been on enough hours that it is time to replace it. If you are running it pretty much all the time, assume it needs to be replaced annually.