If it's a thumbscrew just yank on it while turning to get it coming out.
If you can get it away a bit as in wobbling, take a flat blade screwdriver and jam it between the screw and cover, then pry it out while you turn the screw. Use the flat blade as a crowbar to pry the screw out while turning it. I guarantee that will work. I am an electrician and I encounter this all the time, prying with a flat blade will work for you.
You may have to retap the case threads to get another screw in properly. If it's just the screw that was frigged up then your case may be good.
Note: don't actually pry with the flat blade. Just stick it under the screw then rotate wither way to force it out while spinning the screw with the other hand...