You can go to most any auto parts store to get one of those generic little cube with the various spaced prongs that lock into a caliper to twist them back in and just push it in by hand until you can fit your tool in there. Or if you're lucky enough where your car doesn't have the ebrake built into the caliper you can just push the piston in. If it really did pop out too far, you'll know pretty quick and you would need to either rebuild the caliper or buy a new one.
