Serana disappeared during dawnguard questline.

Solution 1:

First of all, check Fort Dawnguard or Volkihar Keep (depends on whom you sided with), since that's where she should reappear when she gets lost.

Otherwise, if you're playing on PC, you can use the console to either teleport to Serana via

player.moveto xx002B74

or her to you via

prid xx002B74
moveto player

Note that the xx has to be replaced by a number, usually one of 01, 02, 03 depending on the DLCs installed. You can also use help Serana to figure out the correct value.

Should you be on a console instead of PC, you probably will need to convert the savegame to PC, ask a friend with the PC version to do the above, and convert it back. Make sure that friend activates exactly the same DLCs and mods though, otherwise the re-converted savegame might fail to load.