After millennia of imprisonment, the chaos spirit Discord has broken free of his stone prison. Celestia wastes no time gathering the Bearers of the Elements, but before she can retrieve the Elements of Harmony, she falls victim to one of Discord's nefarious schemes, no doubt set up many eons ago to exact revenge upon the Princess, designed to make her suffer as he did for so long.

Or maybe he just superglued her horn to a keyhole.