When my kids get really stuck we do our "hot chocolate" breathing, and then I try to change rooms with them into a new sensory environment. Doesn't have to be a major difference, just enough to jog their surroundings...often it gets them unstuck and we can move through. Drinking water can do that too.
Hot chocolate breathing: child holds imaginary mug of hot chocolate in front of their face...they slowly smell in the yummy hot chocolate and then gently blow on it to cool it down. 3 repetitions usually does it.