There isn't a direct glider analogy to being "in irons" (you can always dive to gain airspeed, assuming you have any altitude to lose of course - not having any altitude would mean you are guaranteed to crash, since gliding is basically flying slightly downhill all the time).

But there are plenty of ways to get in trouble when gliding, most notably flying into an area where there aren't any sources of lift. This means you can't gain any more altitude, and you'll gradually lose your remaining altitude and probably have to land in a field or something.