Serana won’t come out of the tomb/barrow

Were you doing the mission where you had to go there? Have you tried casting Detect Dead to see if she's hiding somewhere? If you aren't doing that mission, she will either be at the vampire castle or at Fort Dawnguard, depending on which faction you chose to help; sometimes she will mistakenly say the glade line after you complete that mission, but she will still return to the base.

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.