Twilight wasn't prepared for this. Nopony was, really. When a strange city with stranger inhabitants suddenly reappears in the frigid north, Twilight and her friends, along with Princess Cadence, are sent to investigate these crystal ponies and attempt to open friendly relations with Equestria. But between competing with other nations' envoys, trying to make friends with the aloof, apathetic inhabitants, and investigating the causes for the city's reappearance in the first place, they have their work cut out for them...

AU tag because this is most assuredly not canon.
Emeral Bookwise and HiveLordLusa both deserve thanks for their huge help with this!