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

The 12 years old Karen falls off her horse, and when she hits the ground, she's way back in the year 528, in England, near Camelot. She's immediately taken under arrest by Sir Lancelot: because of the dark color of her skin he's convinced she's a demon. But with some help of the 20th century's advanced technology she can safe herself from the stake and establish herself as great sorcerer. Only with her help, Arthur becomes as famous a king as we know him.