I thought this would be a simple job... I'm trying to replace a ceiling light fixture and all went well until I attempted to remove an old screw that is located where the bracket for the new fixture will attach. At first, it wouldn't turn at all. I gave the screwdriver a gentle whack with a hammer to jar it free...which worked, except now the problem is that the screw head turns but the screw doesn't come out. I'm concluding that the threads are somehow stripped. Now what do I do (other than replacing the entire outlet box)?? In the attached pic it's the one on the lower right (sorry for the bad photo quality). Thanks in advance.