Disappearance of the USS Enterprise (March 5, 2015)

When Dr. McCoy jumped through the Guardian of Forever and saved Edith Keeler, the USS Enterprise disappeared. She disappeared by dissolving like the USS Defiant did in The Tholian Web. The Guardian of Forever said "Your vessel, your beginning, all that you knew, is gone." Captain Kirk and the landing party found out that McCoy had changed history and, as a result, the Enterprise--and the Federation--no longer exist.