In Biological terms turtle come under the reptiles which is coldblooded animals category. It means it cant be tolerated sunlight which causes the heat stroke.It looks like dry and hot. so you always keep this coldblooded animals away from sun directly
If turtle is overturned and cant correct itself in direct sunlight it will eventually die from thirst, starvation, stress, etc.,it may die from exposure or heat stroke.
A turtle with heat stroke will be dry and hot. Place him in a dish of tepid water to recover. Always be sure your turtle is able to get into shade whenever he wants to. Never place your turtle's enclosure in full sun or place a heat lamp on the enclosure so that the turtle cannot get to shade.