There could be many reasons why she dropped her tail...from the possibility it was caught in the lid of the cage last time you had it open, to a sudden noise in or outside the house the startled her enough to drop her tail, or maybe the shadow of a bird flying past the window.

If she is otherwise healthy and eating normally, there isn't anything to worry about as most wild crested geckos have lost their tails by the time they reach adulthood. They can function perfectly in all aspects without it.
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PHLdyPayne