She should go back to Breezehome. That's where she's supposed to go. If she doesn't return there, it might (at first at least) be the game actually taking the time to calculate how long it would take for her to walk from wherever she is to Whiterun. But more likely is that there was a glitch and you've lost her forever. If I were you, I'd consider that a blessing. I can't stand Lydia, or Lyds as I always call her, usually in the context like "shut the fuck up Lyds," or "Damnit Lyds!" or "when did I invite you into my house, Lyds?"
This is your chance to invest in a better follower!