Not quite understanding you.
If the allen key has rounded off and the allen key no longer grips, or the screw is cross threaded.
If its the former you could possibly cut a slot in it for a screwdriver,or drill the off the bolt, and then extract the remaining stud from the other side. It usually happens when people use the wrong size tool - eg a metric allen key when in fact an imperial one was the correct size.
If its cross threaded you may need to either retap the thread, or resize the fasteners depending on how mangled up it is.