As I see this the most realistic possibilities are, in order of likeliness;
1. She ran off with someone, voluntarily, as I have stated before, for various possible reasons.
2. She was taken advantage of and while struggling she was extinguished (suddenly-not planned) and disposed of, her body will probably be found sometime as a result, perhaps when the farmers plant in the spring.
4. There was some sort of a drug overdose, which would either be covered up by anyone who provided the drugs.
5. She wandered off alone into a secluded field or woods and died due to a medical condition, excessive blood alcohol or a drug overdose. (the weather that morning as I recall, was not unbearably cold, however,contrary to what others here have stated)
6. A skilled predator planned her abduction, luring her, based on the condition and ease of which she may have appeared as a victim, in which case her life was extinguished, her remains could be anywhere in the U.S.
7. She was kidnapped for the purpose of "human trafficking".
8. She was abducted for sexual reasons and possibly held captive by an older man for use as a sex slave.
Forgive me if i sound insensitive, but this is Reality and at some point Everyone will have to consider all of these possibilities.
If I have left any possibilities out, please feel free to present other considerations, hopefully in a concise manner as I have done above.