Try a hair dryer on low to gently warm it, not too close. If it begins to become pliable, heat it a little more, then place it between two flats with a little weight on top and let it cool. (Like cutting boards and a can of veggies)
I used to save some LP's that way after they caught a little sun. Vinyl is different then the Plasticville material, so be cautious when you test the pliability.