Soul Crain Beyond Death Quest (need help !!!! ) Bugged

[ٴ ٴ ٴFerv]

Hello i got a problem with my game... i am doing the quest for the dawnguards with the vampire Serana on my side , but the thing is .. i dont have her anymore and she is not following my anywhere , i went to find her mother Valerica but there is no interaction !! i got that dead horse also if that mathers ... so Serana is no more at my side anymore and not even if i exit the map and go back in , nor that i w8 24 hours and hoping on her return .. i never seen her dieing and i know she is not dead , how do i solve this ?