I've done the research and have seen the first option for what might have happened, your follower got stuck somewhere. All you need to do for this is backtrack, or wait in your house for three days...Okkaayyy
The next thing that can happen is that there can be instances (cause we allllll know Skyrim is a helluva glitchy game) that your follower just downright disappear, we'll simple I can just use the set follower count command... Not really, see I like my Xbox so that's what platform I use, don't laugh...
The third option that I've found is that somehow your follower has died. I nearly leaped for joy when I found out that you can find the body in the hall of the dead. Low and behold in the Hall of the Dead in Whiterun, good ol Uthgerd the Unbroken's grave is there. Great now I know for a fact I can get a new follower because she is dead...Still nope... Please let me know if there is any other questions on the subject.
Thanks, Happyfacejunior