Serana disappeared [duplicate]

I am doing the Dawnguard questline and I was getting the last elder scroll from septimus sigmus.

I told her to wait while I was doing stuff with the lexicon as she kept getting in my way. She did however follow me out when we were finished we then went to the dawnguard to talk to the moth priest and we now have to find a moth priests knife.

I went to Riverwood to feed and I'm not sure if she went there with me or not but I have not been able to find her at all throughout the rest of my game play. Nothing on google has helped me it's like she has just vanished from the game even though I still need her for quests.

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.