How far in is the screw? Is it a phillips-head screw? What kind of hand tools do you have?
There is a tool called an Easy Out:
http://xml.nextag.com/Irwin-53402-2-...AEA0AF29C746C3
That would be the right way. Lacking those tools and skills, you could try the following crude methods:
Can you grab the screw head with a pair of pliers and back it out? If not, maybe you could use a dremel tool to cut a new slot for a flat blade screwdriver. If that doesn't work, use an electric drill with a bit that's just a little larger than the shank of the screw but smaller than the head of the screw, and drill down through the head of the screw until the head comes off. Then you should be able to remove the plate, and grab the screw with pliers. Throw that screw away and get a new one.
When using any of the techniques, be very careful you don't run a sharp tool across the finish of your bass. Or through your hand.
When you put the new screw in, make sure you use a NEW screwdriver, so the edges are not rounded (don't use the same one you used the first time

).
Ed