I had Mjoll as a follower(and wife) and we went into Blackreach. I discovered Crimson Nirnroot, and went crazy over them. So I searched for them. To increase my effectiveness, I cranked my turtle beach, shut off all sources of music, footstep noise etc. I took off all my clothes and equipped fast heal spell to increase health and stamina. Mjoll starts annoying me with her dialogue that I've heard a hundred times(has a habit of talking when I'm talking to important people) so I got upset and told her to wait. I found all my nirnroots in about a half hour's time, without a guide.(accomplishment for me, I svck at finding stuff.) After my feeling of joy died down, I realized Mjoll was missing. I was cursing myself because she is somewhere in Blackreach and I can't find her! I spend 15 minutes looking but I gave up. All I know is that she is outside, and not in anywhere like a derelict station or anything. Is there any possible way to send her home or something? Is there any way in the main quest, or any side quest which forces companions home? Or is she gone forever??

After a while, she'll go home on her own. Not sure how long it takes, though. You'll get a message saying that your follower gave up waiting for you and returned home.

If what you said is true, I will drink to your name!

Fast travel finds them for you. Once outside just fast travel to where you are (redundant, yes) and followers, dogs (if any) and horses (if any) will reappear.

If you are playing on PC,

• Load an earlier saved game where she is near
  
• Bring up the console with ~
  
• Click on her
  
• Write down the code for her which I later refer to as ThatCodeYouWroteDown
  
• Then load your latest game.
  
• Bring up console
  
• Type this: prid ThatCodeYouWroteDown
  
• Then type this: moveto player
  
• Then exit the console and she should be near.

thanks xoham, that will come in useful when I get the PC version.(I usually get console first, pc after) IT seems that Seseron is right, Mjoll just gave up and went home. Ahahah! That's cool!