My thief was having trouble getting through Dawnguard quests with dragons swooping down on me constantly, so I detoured to do the main storyline so I could unlock Dragonrend.

Before reading the elder scroll at the tear, Serana was following me and everything was okay, but when I came back from the tear to fight Alduin with Parthurnax, she had disappeared. I carried on with a few quests more afterwards, having lost her before and figuring she'd show up eventually, but she never did.

Now I'm at the part in Dawnguard where I have to speak to Serana about her elder scroll. A quest marker is hovering over where Serana should be in the Dawnguard castle, but underneath, there is nothing. If I target her using PRID and use the moveto player command, nothing happens except that the quest marker moves closer to me. I've also tried using the placeatme command with her baseID, but that spawns a generic Serana that doesn't have anything to do with the quest.

I've also tried deleting and reinstalling Dawnguard, as a last ditch effort. I am not running any mods except for Skyrim Crash Prevention.

Can anyone think of anything else I can do to get Serana back?

Ah, that happened to me with Aela last year, looks like they haven't fixed it. Reload before reading the scroll and entering the tear, dismiss Serana then do it. If you are on pc, you can type in her id and see if she is anywhere, but with me and Aela, it brought me directly to the tear where you will see no Serana...

Damn, I don't have a save that far back. If that's the case, I guess I can't complete Dawnguard on this character. What a shame.

I think I'll pack it up on Skyrim until the next patch release.

It happened to me. Wait over 72 hours then she will return. This fix works and it is a bug.

http://www.uesp.net/wiki/Skyrim:Serana