If, on the other hand, you're talking about the flap inside the water reservoir, they also fatigue over time but are a cheap replacement from again, hardware stores.
That is, if it's not simply that the chain hasn't gotten stuck on something.
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At some point in the life of all flexible products, replacement is the only option. If it's not returning to a cup shape after use, it's compounds may have dried or otherwise fatigued to the point where a new one is due. The fortunate thing is that plungers aren't terribly expensive, and they're commonly available at hardware outlets.
Sorry, if heating it in warm water does not work to restore it, you have to assume the manufacturer designed it to fail in order to sell more plungers..
Buy a different brand next time :-/
I am not sure I am picturing what you're describing.
My mental picture of what a toilet plunger looks like, is consistent with the pictures in the Wikipedia article for "Plunger", here,
Basically a plunger is a rubber (or some kind of elastomer) suction cup, on the end of a stick.
Again, I don't know what you mean by "stuck in the up (open) position".
Maybe you're saying the rubber suction cup gets bent backwards on itself, or something like that.
I am guessing it will not take much force to bend the cup back to its original shape, since it is made out of rubber, or something like it. You could probably bend it back with your hands, if the cup were not too dirty to touch.
Assuming you want to avoid touching the cup with your hands, because it's covered in shit or whatever... maybe snagging an edge of the cup on the rim of the toilet bowl, and then pulling on the stick end of the plunger.... Maybe that would provide enough force to pop the rubber cup back into shape?