Ah yes, happens many a time...
Can you get a pair of pliers onto the top of the screw or is it recessed?
Will a blade screwdriver give you some pruchase on the screw head and allow you to get it out?
It might need a spray with some CRC or WD40 if its bound up.
If none of that works, you might be able to cut a new slot in the top with a dremel or it might come to drilling it out.