I have an aluminium baking plate that I did put in a very hot oven and now it is soft, so I can bend it or almost roll it up. Why?
I don't know whether or not this is what happened in this case, but some metals have multiple crystal (solid) states. Changing heat changes the crystal structure, which changes the metal's physical properties, such as stiffness. Tin is a good example - if cooled down cold enough and long enough, it becomes very weak, crumbling under the slightest pressure. It could be that you heated up the pan hot enough to do something like this. Again, I don't know if this was what happened, but it is possible.