Stuck in the Ancestor's Glade

Load an earlier save or use the console command "tcl" to walk through the gate.

This glitch occurs due to you not having Serana follow you during this quest. From this thread:

When you have to go to the Ancestor Glade to read the Elder Scroll on the Dawnguard side, if you DON'T have Serana following you and have her wait somewhere, a glitch follows. When you enter the glade, Serana will automaticly appear at the start of the cave, but won't move. When you talk to her, she's already "following" you, but she isn't because she's standng completely still. I continue, thinking she'll catch up. I go on and read the scroll... Objective updates to tell me to "Speak to Serana", problem is, the door leading out is barred up, with no way out. Completly sealing you inside the cave. So unless you have a back-up save before this happens, you're stuck there for all time.

tl;dr: You're going to need to reload your last save.

Actually I stuck at first too and couldn't find Serana but then I killed all vampires behind the gate ( using Sanguine rose stuff) and when last one had died the door has opened and Serana got back to me . Hope it helps :)

My issue was that my saves didn’t go back far enough so i did the save right as i was entering the cave. If you go back to where you told her to wait. Then she will be there and will start following you automatically again. Then go back to the cave and do the quest

I found that you can attack Serena until she becomes an enemy and have her follow you into the Glade. You can fight her until she is down, and waiting an hour works to get her in the Glade with you without having to go back to other saves.

I found if you are levelled high enough, if you get the right angle you can use vampiric grip to glitch her through the gate, then revert form and sheath weapons and agro should drop.