Princess Sunshine Is Dead

Princess Celestia forgets that she is, in fact, a princess.

Everypony knows that Celestia is the princess of Equestria. She's been one for as long as anypony could remember, and she will probably be for generations to come. Some would even argue that there is no 'Celestia' without the 'princess' before it.

And yet, this morning Celestia has no idea why she is a princess, and her subjects have no idea why she isn't being one.