Pull the 2 screws securing the coil, disconnect the flipper, take the plunger and link/pawl with you to any local hardware store and pick up a roll pin for that hole. If you can't figure out which one ask a clerk to help you. They might be able to push it in there for you or hammer it back in on a hard surface when you get home. If hammering doesn't work (or is proving difficult) use a C clamp and you can slowly press it back through most of the way and finish with a hammer (and a nail set if you have one).