I was doing a quest and told her to wait...that would be my only guess as to why she's no longer with me...I know she didn't die or anything like that. However...as much as I've looked for her I can't find her..I don't want to use console commands to summon another...that just well...wouldn't be satisfying. My problem arises whenever I try to get married, I go to the priest and arrange my wedding, I'm told to show up the next day..so I do, guess who's there? Lydia and for some reason the bride to be tells me to buzz off and the wedding gets failed. I think if I could find lydia this might solve my problem...Any suggestions?
if you are on the PC you can find her Ref ID by going to the elder scrolls wiki and searching for Lydia. It will say below her picture most likely. Then hit the tilde key ~ and type player.moveto (the refID) and hit enter. it will teleport you to her. You can see where she is at. It did this with Faendal for me and it messed up trying to get married to Camilla in Riverwood. I just had to abandon marrying her and went for someone else :\