I'm very fond of Rubbermaid Premier food storage containers, mostly because - unlike my old plastic microwave containers, they don't stain when you heat tomato-based food in them.
[Not a paid endorsement. I honestly like the product. Pricey, but durable.]
But the other day, I heated up some food, but I didn't get to it for a while. Unfortunately, the food cooled off, created a vacuum in the container, and sucked the lid into a permanent reverse-convex shape.
Taking a hint from this forum, I tried reversing the process by boiling water in the container with the lid on tight, hoping that the steam pressure would pop the lid back into shape.
Didn't work. Still reverse-convex.
However, the lid was still warm & pliable, so, as mentioned in the forum, I put a plate on it to press it back into shape, and then put something heavy on the plate.
I let it cool off, and now it's back to normal.
Hopefully it will work for you, too.
6 hours ago