Depending on the age of the light it could be a couple different things. If its newer it could be the ballast, if its older it might just be a starter. When you turn the light upside down do you see a small round silver, or white object protruding from the light anywhere? if not then the light has an electronic ballast built in. You can also change this but it will require dis-assembling the light. If you can post a pic of the underside of the light, then we would know for sure if it has a starter, or ballast. It could also be something as simple as the on-off switch, or a loose wire.