You were sent to go find Serana, she was found asleep in Dimhollow Crypt, as you approached to wake her...

"Unh... where is... who sent you here?"

She looked up at you, still dazed, as she seemed to be unconscious for a very long time prior to this, needing to adjust once again.

(I apologize if any information is wrong, as well as this disclaimer, I nearly never get Serana, or do anything really in Dawnguard, so I don't know much about her character).