Or maybe you didn’t forget but because of the heat and maybe the strong smelling food that was in the cooler, it now has a nasty smell.
Don’t throw it out, it can be saved. Probably.
Make a paste of baking soda and a little water and scrub out the cooler. Dry with a soft cloth.
If the smell is still there, add a teaspoon of vanilla extract to a few cups of hot water. Sponge the solution on the surface of the inside of the cooler and let dry.
You should have a sweet smelling cooler but if you don’t, after the vanilla solution, let the cooler sit in the sun a few days.
By now, if the smell is still there, go ahead and retry all the steps. If after that, you still have the smell, it’s hopeless, go ahead and chuck the cooler.