I dismissed Serana to do a mission on my own and i went back to Dawnguard and went to ask her to follow me and there is no dialog for it, the only dialog i have now is...will you change me into a vampire and i need some of your blood for my arrows. Is there a known fix for this? im also on PC.

On Steam Workshop (and Nexus) there is a mod 'Toggle Serana Existance'. What it does is allow you to more or less switch her on and off...when you toggle her back, she conjures beside you, and it seems to reset her which fixes this bug (did for me, anyway).

I have had this on Xbox. I'm not sure, but I was able to trigger the glitch when I went back to the undercroft at Castle Volkihar. This is after the main DG quest.... I found no fix other than to reload an old save. I find having a boatload of saves is my best armor for bugs!

This is a known glitch with no workaround that i know of on 360 cannot say for the other devices.

Thanks i found away to resolve this using console on a pc ....

Unlike followers from the vanilla game, Serana's dialogue does not check the normal follower quest, but certain quest variables in the quest "DLC1NPCMentalModel".
You can take a look at your Quest variables with the console command "sqv DLC1NPCMentalModel".
The following conditions must be met, for her to be available as a follower: CanFollow_var = True / IsFollowing_var = False / IsWaiting_var = True / TurnOffComeWithMe_var = False / LockedIn_var = False
In my case the glitch was triggered by the LockedIn_var being set to True.
You can change the variable by typing "setpqv DLC1NPCMentalModel LockedIn_var True".
You should check all 5 variables in your game, by scrolling through the console with the Up- and Down-Arrow keys.
--http://www.uesp.net/w/index.php?title=User:Geistertanz&action=edit&redlink=1 18:52, 13 August 2012 (UTC) (Lurker here) that actually worked for me. I set LockedIn_var to true and voila! --Abdul 10:16, 16 August 2012 (UTC)? http://www.uesp.net/wiki/Special:Contributions/212.118.140.24 Alternative Fix: I was having problems getting Geistertanz' method to work when Serana was stuck waiting after I had told her to move to a spot. My current location at the time was Darkfall. Flipping Lockedin_var did not produce the desired result, but flipping "CanBeDismissed_var" to "True" did the trick. It allows you to dismiss her (on a sitenote, it appears she has recorded dialogue for this event despite it not normally being possible to dismiss her at this point) and then subsequently flipping LockedIn_var to False will allow you to immediately recruit her again and thereby resetting her wait/follow state. It should be mentioned that I had to tell her to follow me twice for some reason before she actually did so, but I'm unsure if this was just a hiccup or if it will be the case for everyone. If necessary, just hail her twice in a row and tell her to join you. TL;DR: "setpqv DLC1NPCMentalModel CanBeDismissed_var True", Hail and dismiss her, "setpqv DLC1NPCMentalModel LockedIn_var False", hail and recruit her again. May need to select recruit option twice. --http://www.uesp.net/wiki/Special:Contributions/81.167.250.34 14:37, 1 September 2012 (UTC) This worked for me. I had dismissed her after the main questline was complete. Both LockedIn_var and TurnOffComeWithMe_var were true in my case. Waiting, however long, did nothing. --http://www.uesp.net/wiki/User:Elmindreda (http://www.uesp.net/wiki/User_talk:Elmindreda) 16:56, 8 September 2012 (EDT)