From Leopard Gecko Wiki
Like many Geckos and Lizards, Leopard Geckos have the ability to drop their tails. Leopard Geckos usually drop their tails due to rough handling, as a defense mechanism, or cage mate aggression. When a Leopard Gecko re-grows its tail, it usually does not look like the original tail, as seen in the picture to the right or below.
Tail Loss Care
When dealing with a tail loss, to ensure proper growth and to reduce the chance of infection, Neosporin without pain reliever should be rubbed on the wound once a day until the wound has healed.
Also if a Leopard Gecko who has lost its tail is housed with other geckos, it should be separated until it has fully healed.