Thats pretty weird if you have any older save maybe have a look and see if the same thing has happened. If you finished Dawnguard then Serana (if she isn't cured) is the only way to get the Vampire Lord form, If you've sided with the Vampires she should be at Castle Volkihar and people say she can be found in Darkfall Cave for some reason, also that she can sometimes go off wandering (but should return to the castle in a day or two). Unfortunately though depending on where and how you dismissed her she can sometimes just vanish from the game.
If your on PC you could try console commands to return your powers (need to know your race if you not a Vampire at all anymore) or teleporting Serana to you with -
prid XX002b74
moveto player
The XX needs to be replaced with the load order of Dawnguard.