yea i was wondering about this. My mother washed one in the machine and it obviously went massive. Let it dry out and now some of the crystals have bonded (for want of a better word) together in hard lumps.
I just smacked mine on hard surfaces until it loosened up again. Seemed like the logical thing to do. You could maybe punch it real hard, too, but I found smacking things with the headband to be a winner.
not used one for a while but i used to hand wash mine in warmish water after it had been used then dried on one of those racks that you hang your bath towels on the radiator with....dried out fine.