First, try reseating the lamp. Often times, this can be a remedy. To do this, plug the unit and open the lamp door. Remove the lamp by unscrewing the housing, pulling it out and firmly pushing it back in. Reattach the lamp housing with the screws, and then reattach the lamp door. In short, you'll be removing the lamp, and firmly re-inserting it.

If you're not comfortable doing this, you may want to have a technician inspect the unit to give a full diagnosis.