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This may mean that the overheat has affected the non-stick coating itself. Otherwise the stain would not have formed in the first place. One possibility is that the ironed fabric was a low temperature plastic material that melted and stuck to the iron.
Get ready a detergent solution. This can be the same clothes washing powder that you use or a liquid hand wash soap diluted with four times water. Heat the iron to a warm temperature not too hot to touch, turn it bottom up, pour a few drops of the soapy water on the stain. This should not boil if you have kept the iron just warm. Use a plastic (disposable) spoon or plastic knife and gently scrape from the outside edge of the stain inwards. If you are lucky the stain should come off (provided the non-stick coating itself was not damaged).