If you notice you lost a companion while wandering around,choose to wait for an hour and a lot of times they will show up. If that doesn't work, go back to the Jarl in Whiterun. If you don't own a house, and she isn't dead, that is where she should be. You just may have to look around for her since she can wander around the place. Companions will generally go back to where you first pick them up if you lose them.
If she isn't there, chances are she died somewhere along the line and you'll have to reload an earlier save.
Lord of Lurkers
Guess what? I got a fever, and the only prescription is more cowbell!