Why won't my follower, Serana, follow me?

Billy Mailman's answer is rather complete, but just simplifying things, the following worked well for me:

If you are playing on a computer:

1. Open the console
2. Right click on Serana OR type prid 02002b74 and hit enter to select Serana
3. Type Setpqv DLC1NPCMentalModel Lockedin_var False
4. Hit enter key

This will cause the "Follow me" dialogue option to reappear.

This solution was found on this link.

The list of bugs associated with Serana is rather large. She's the single most complex NPC in the entire game, with an entire hidden quest (called DLC1NPCMentalModel) acting under the hood to control her follower behavior, her dialog, etc., independent of the stuff that goes on as part of the actual quests you see.

Some of the more common bugs involve that quest getting into an odd state; namely, CanFollow_var, IsFollowing_var, IsWaiting_var, TurnOffComeWithMe_var, and LockedIn_var, which control whether she can follow you, whether she is following you, whether she is waiting, whether you can ask her to stop waiting and start following again, and whether she can be dismissed as a follower, respectively.

If you are playing the PC version, you can use the console to see the current values of those variables using the command sqv DLC1NPCMentalModel. It will list a huge amount of data, with potential spoilers, but hitting Page Up and Page Down should let you browse for those five. Then, setpqv DLC1NPCMentalModel <Variable Name> <true or false> should let you change them. Most likely, either IsFollowing_var got turned off when it shouldn't, or IsWaiting_var got turned on when it shouldn't, but I can't say for certain. It should be simple enough for you to figure out what they should all be set to, based on your own progress through the Dawnguard quests.

If you're on the Xbox 360 or PS3 versions, that obviously won't work, because there's no console. You may be able to get her to resume following by travelling somewhere, and waiting until several days have passed in game.

Additionally, if she's stuck in her combat pose, she may be able to be reset through either knocking her around a bit with the Unrelenting Force shout, by using a powerful Calm spell on her, or else by using (on the PC version) the disable and enable commands on her, to reset some of her AI.

This means that you have a bug, unfortunately this is a common bug and it has happened to me before too, you have to load up a save that is before the start of the ENTIRE questline. However if on PC you can find a script code to skip that mission

You need to either talk to her and tell her to follow, or continue with the quest and when it finishes, then you can get her back again.

My Serana was bugged in quat position and i tried the console tips but it didn`t work. After some time i killed her, she went wounded then i shouted her with unrelenting force and then she debugged! try that!