A Connecticut Yankee in King Arthur's Court (1989)

When 12-year-old Karen hits the ground after falling from her horse, she is sent way back in time to the year 528 to England, near Camelot. Immediately she is arrested by Sir Lancelot--because her dark skin color has convinced him that she's a demon. But with some help from from the 20th century's advanced technology, she can save herself from the stake and establish herself as a great sorcerer, and it is only with her help that Arthur becomes the famous king who is still known today.