Go to the AutoZone/PepBoys.
There's an 'extraction kit' available, based on the size of screw or bolt that's broken.
The kit is a drill bit, a reverse thread tap, and a reverse thread bolt.
Grind the remaining head flat to get a decent surface to drill into, then drill a pilot hole.
Reverse tap the hole; usually, within a few threads the broken fastener will start unthreading and come out.
If it doesn't, you simply continue to fully reverse-thread the hole then use a wrench on the reverse-threaded bolt from the kit.
edit - PS:
If there's enough remaining above the surface, you can spray WD-40 or PB Blaster to free the threads and:
1. Hack-saw a slot to turn it into a slotted screw and back that out; or,
2. File the sides square to take a wrench to it; or,
3. Try Vise-grips.