happens all the time on used bikes . is there a stub sticking out or did it break off flush? if theres a stub, soak in penetrating oil for a day or two. try to grab with vise grip or stud remover[special tool]. if will not move apply heat with heat gun or propane torch , more oil , should come out. if broken off flush, more involved, use pentrating oil as above, drill hole in stud, use easy out. be carefull, dont over stress the easy out and break it off in the hole. if you do break off a drill bit or easy out in hole , you need a small carbide bit to grind through it .if all this doesnt work you can drill out the stud slowly going up in size till just the threads are left . then take a small pick and pull out the threads. failing that and you screw up the threads.[ it happens] you need a helicoil repair . thats the whole thing in a nutshell