Most hardware stores carry screw extractors (you first drill a hole in the broken screw, and then use the screw extractor to remove the screw). These work best on large diameter screws, and may not be an option is the screw is small.

If need be and the screw doesn't back out with an extractor or as bulldog described, just remove all the screws on one side. Once a side plate is removed, the pillar with the broken screws will have enough of the stub exposed to grasp with forceps, needle-nose pliers, or some such. I would put some penetrant on all of them before trying any turning, so you are less likely to break or gouge another screw head, or break off the stub. Worst case scenario would be having that one pillar in a vice and drilling out the screw. New ones aren't hard to come by from a junk or a hardware store, 3-48 I think, but you can measure the threads at any store that sells hardware. When reassembling use a little mild formula threadlock, or the hackers nylock, a snip of 6X of tippet material or plastic food wrap.