Your companion must be stuck somewhere during the mission, if you wait a couple of days, you should get a message telling you that your companion got bored and went home.
Then, you should be able to recruit a new companion
gaming.stackexchange.com/users/11089/Jupotter
This can happen when you get a quest where you have to meet someone somewhere.
Just do that/those quest/s and any
you'll be able to have followers.
gaming.stackexchange.com/users/18965/System Folder
Try dying and then see what happens when you come back.
If that doesn't work, try turning the game off and reloading it.
If that doesn't work, just go back a save point (if it wasn't too far).
gaming.stackexchange.com/users/15323/Brent
I would go to the place where your companion would be if they went your active companion.
Perhaps they will be there and it is a glitch that makes the game think they are following you but aren't.
If they are there I would try to recruit them and see if that fixes anything.
I would wiki or google search to see where your "lost" friend is located when they are not following you actively.
I hope this can help you.
gaming.stackexchange.com/users/14985/Zachary Cash
If you can remember his name search for his name on the site below and figure out his character code.
Here is all you need to know about console commands.
Open console and enter player.placeatme
characterCode
(enter his code you found instead).
This should get him to you,if he is dead enter prid characterCode (to select him)
than write Resurrect after you can use the first code to bring him to you.
Note : You might need to use Disable command to remove some things from game completely or use moveto to get to them.
If something is still wrong read the wiki good
there is bunch of commands to do almost anything,just figure out what will work best for you.
gaming.stackexchange.com/users/9888/Mentales
I had my companion with me on the mission "Forsworn Conspiracy" and after completing the mission they vanished.  I figured 'oh well, I'll go get a different companion', but when I tried they were all saying that it:   Looks like you already have someone.   I tried fast-traveling and waiting to no avail.  How can I fix this error?
How do I get a companion when it thinks I already have someone but I don't?
gaming.stackexchange.com
<the-elder-scrolls-5-skyrim>
gaming.stackexchange.com/questions/38558
gaming.stackexchange.com/users/15326/irichey25
Wait a couple of days and your companion will get bored and return home. You can also try dying or reloading the game. It seems that the companion can glitch to a location. The community details few possible fixes.
Few options are proposed, regarding the issue.