Has it sheared off or is there still some of it protruding???
Pics would be good.
Is you still have some thread showing, is there enough space to get two nuts?
If so put one nut on top the other... i.e. wind them both down the thread and then hold the bottom one in place with a spanner and tighten the other nut onto the top of it to lock them togther, you should be able to undo the stud by putting a spanner on the lower nut and winding it anti-clockwise.
- use plenty of WD40.
If the stud has sheared, then you have two options.
If there is a bit of the thread protruding (i.e a few mmm) then you can weld a nut of the same size (or slightly diameter) on top of the thread - thus you can use this nut to undo the stud.
Or if no thread showing...
You need to grind (or file) the top of the sheared stud completely flat if possible and then find the centre of the stud and centre punch it.
Then drill the stud out....
Start off with the smalllest drill bit you have and drill all the way through the stud... then just keep going up in drill size until you get to the tapping size size of the studs (or very close). If you have drilled the stud centrally then you should be able to pick the remaining bits of the stud out of the thread... then just re-tap the hole and fit a new stud.
If this doesn't work then you may have to drill the hole oversize and tap the next thread size up... e.g if original was M6 (6mm thread size) drill the hole 6.75mm (or 7mm) to tap for an M8 stud.
Ian