Quote from Pestilance on a different thread.
Ok, so for some reason during one of the first Companion missions (the one you become a member after completing) I lost my merc. I went back to Whiterun to see if she was there, and holy crap, there stood Jenassa w/o the full set of enchanted elven armor I had given her. I must have forgot to un-equip my mara amulet, so when I talked to her (had the option to hire, even though she still had the sword (looked the same as the unique I gave her) for 500g) she wanted to get married.
After all of two sentences, I quick traveled to the temple, talked to what's his name, set up the marriage and waited. I've literally waited the entire time at the temple, and eventually get a 'failed' mission. No matter what I do I keep failing to get this done, I've read it happens within or close to 24 hours. After whats his name tells me that the ceremony will begin at dusk and end at dawn I literally just sat there, and keep getting a fail.
Not only that, but the first housecarl I got (Lydia) who I accidentally killed appears on the floor next to the door after it tells me I failed an all the npc's tell me to get out. I go back to Whiterun and the merc (Jenassa) is gone.