Mudguard deformed due to heat
A heat gun is best for shaping plastic, but if it deformed in the sun then a hair dryer will probably work too. Depending on the shape you may want to have a heat-resistant surface on hand to press it against as you heat it; in my experience plastics can become a bit uncooperative and bend in strange ways when heated. Good luck!

Citation: My personal experience from working with reshaping a variety of plastics.