Nothing. It will heal, but not completely straight. The cat may need pain medication. If there is nerve damage the tail may be limp. Sometimes this recovers with time.
If you think your cat has broken its tail, take them to a vet immediately. They may need to receive antibiotics if they have broken skin or signs of infection.
For a serious concern visit your vet ASAP. Try to make sure the tail is straight as possible, and keep her indoors until healed. Avoid touching it if possible.
