Hey guys, I am very lost and confused right now. Serana, my current follower, has completely disappeared. I was just building my Hearthfire house, when i noticed Serana was not following me anymore. I never asked her to return to her home, but in needs, I went to Castle Volkihar to see if she was there. She wasnt. Then to see if she had died or something, i asked lydia to follow me, but she said "It looks like someone is already following you". Then I tried to see if i can fast travel and she will show up. she did not. Please help me guys, I have a lot of valuable stuff on her. THanks

It's entirely possible that you told her to wait and neglected to command her to follow you again. Trace your steps back to where you were prior to building your house--worst case scenario, you revert to an earlier save and lose a few hours of gameplay. On a side note: Serana can't die as she's an essential NPC.