It’s just like 1,000 years ago! Because back then, a crystal princess was petrified into a statue and overthrown by a greedy and evil tyrant. But now is different. A different crystal princess and a different evil king.

@SolidSonicTH  
That’s not fart gas. That was just green mist that would petrified you.

With Cadance petrified to an obsidian statue/totem, Shining Armor and Flurry Heart who were about to got to Canterlot with all the other Crystal Ponies for the Friendship Festival will be shocked when they see their Crystal Princess petrified on the Storm King’s communication potion projection/screen and captured, or imprisoned, or punished, enslaved by the Storm King’s forces. And when the Storm King arrived in the Crystal Empire, he discovered that there’s a fifth alicorn which is Flurry Heart. The Storm King thinks he could let Flurry take Twilight’s place instead. He pulls out and obsidian orb to petrified Flurry. But Shining Armor told him not to because Flurry is not the true alicorn princess even though she was princess of the Crystal Empire. So instead, the Storm King imprisoned them in a cage. So I agree that with Cadance petrified into stone, the Storm King and his army would enslaved every Crystal Pony of the Crystal Empire just like what King Sombra did 1,000 years ago. The Storm King’s forces would also punished or imprisoned the Crystal Ponies as well. And the crystal heart was fading due to the storm clouds blocking the skies above the Crystal Empire and the Crystal Ponies would presumably be on low spirit and lose the crystalline colors of their coats.  
Note: This is all my opinion of what happened during the movie because the Storm King wants to conquer all of Equestria. And also, I bet Sunburst was part of the Friendship Festival as well but imprisoned/enslaved in Canterlot. The crystal guards of the empire were captured as well I believed.

Why did I think that was fart gas?

‘Alexa, Play Who’s got your love by Stonebank’