yes, that's a broken riser. You can unscrew that threaded bit. It's not part of the sprinkler.

You will need to replace the broken riser underground.

If nothing else is broken, you should spend $1.xx and buy a Hunter swing joint, or something similar, to connect the sprinkler body instead of a fixed riser. Because of the swing joint flexibility, it won't break when you run it over with the mower.