A similar thing happened to me on my G-Force coilovers, but with mine the stud snapped. I took the coilover back off the car and managed to get the remainder of the stud out of the topmount by putting the topmount in a vice with a long 19mm socket over the stud head (i.e. the underside of the topmount). By doing the vice up, the stud was pushed out of the topmount and into the socket. I replaced the stud using the same method but with the 19mm socket on the top face of the topmount.