If it says PGP on the top of it, it's a rotor, and not a spray head. If this is the case, then if you extend the head up out of the body, it will have a red or gray nozzle in it, and it will have a small number on it indicating what size nozzle is installed in the spray head.
This one broken head shouldnt prevent the entire system from working, unless you only had one zone for your entire system.
Replacing the head is a relatively simple task. If you dig the head up, it's most likely attached to the PVC laterals with a flexible joint. You can unscrew the head from this, and re-attach the head. If you go to Hunter's web page, they have directions for how to adjust heads.