Let see how well I can explain this. I have 2 tribes in my vv2 game. One is mine and one was my daughter's. This is twice this has happened to me now. I went to get into my game and say 'good morning to my little people and I noticed it was not my tribe. I thought Ok, I forgot to go back to my tribe after capturing the _bug_ I saw in my daughter's tribe. http://michael.myultimatespot.com/vv2.html
Well I went to change tribes and saw that mine was gone. Gone I tell you. Poof! No more. Just an empty space that said "NEW PLAYER".

I was on a roll. I had one last collectable that I have been waiting for, for several hours of several days. I am lucky that I found and copied the LDW folder in My Documents So all I lost was about 3 weeks of work. I suppose the moral of the story is I need to copy that folder more often.

Why do I lose my tribe?

The usual reason for that is a corrupted savegame file. Savegame files can become corrupted for a variety of reasons, including computer crashes and powering off while the game is running. I can't say definitively in any particular case without more details, such as what files were in the game's save folder, what the date/timestamps were, etc.