How to remove a sheet metal screw from the threaded end
If there's enough sticking out that you can grab on to, a pair of locking pliers would likely help turn it but it will be hard to fully unthread since there will come a point where the pliers don't have enough to grip on to.  If you're lucky, you might get enough clearance to cut it or turn it from another part of the screw.

Another option is to simply drill it out and then tap new, slightly larger threads to use with a larger screw when re-installing.