My husband fixed mine. I had tried sitting it in the sun with heavy books on it, but I got creases where the books were. He said my mistake was putting several objects on it instead of one big flat thing.
So he set it in the sun, on the hot patio concrete and put a heavy piece of plywood type stuff on it. I forgot how long he left it there...at least several hours and maybe overnight.
It is fine now. I remember googling the problem and finding many solutions. You might try that also.