In the quest: "At the Summit of Apocrypha" I have reached the stage of getting the command "E ride Sahrotaar".

I go thru the mounting action, and the dragon flies away.

But I am left behind, in a riding pose, floating in thin air.

How do I solve this problem ?

activate a console command that allows you to fly and then follow the dragon.

Try one of the solutions here.

http://forums.nexusmods.com/index.php?/topic/909819-dragonborn-dlc-dragon-riding-glitch/page-1