Try fast traveling if you haven't already. That should make her appear unless she's bugged out in some way. You could also try waiting/resting a really long time. You should get a message that "your follower has grown tired of waiting."
Finally, if none of that works, try doing a quest where you have to take on a follower. They should replace your previous follower.
Even finallyier, if on PC, look up her ID and use the "placeatpc" command to bring her next to you.
I've tried fast travelling, and waiting. Nothing. Usually it worked too.
I have a quest where a follower follows me, though, I can't equip him. It's the guy I have to deliver to the cannibals. He and her have been questing with me for the last 15-20 hours, and then out of nowhere she just... disappeared.
And yes, I'm on PC. How do I look characters up on console though? What's the key word? And do I just type her name after? Or..