If there is any kind of shape left in the screw head at all then try a slightly bigger allen key, even if it is metric an imperial may fit tighter.
Otherwise, I have also done what Mr Stokecity suggests - the Torx bits can "Grab" any trace of corners well.
If you've got a completely round hole, then you will have to either cut a slot with a hacksaw to use a screwdriver - Or use a centre punch like Tom suggests. The only problems with the punch is that it could slip, and damage the frame, or if you don't hit the head on the side enough (to turn it) you could damage the thread in the body - but if done right - It will work.
Cheers - and good luck