Rapunzel's magic hair plays an extremely important and crucial part in the plot of Disney's Tangled. Rapunzel's hair is what makes her special, glowing bright gold when she sings a special song known as "Healing Incantation." It is known to have the ability to heal the sick and wounded as well as return those who have died to life.
Her hair magically grew back in Tangled: The Series, but it had lost its healing powers.
In the TV series, Rapunzel's magic hair grows back to the length and color it was in the film. It grows back when Rapunzel and her handmaid, Cassandra, discover some mystical rocks with magical properties during a secret adventure. In the series, her new hair is revealed to be indestructible and cannot be cut. It's later revealed that Rapunzel's hair no longer possesses it's original powers to heal, but is revealed to have acquire a new power, protecting Rapunzel by creating a magical shield.