@GOMAR and others who don't believe there are any lost tribes, I suggest that you do a little research such as the following book:
The Unconquered: In Search of the Amazon's Last Uncontacted Tribes
By Scott Wallace
Narwhal I think it would depend in what context one uses the word lost.
Literally lost = can not find it, however there are tribes that are known about but continue to live as they have done for thousands of years in isolation, but the possibility of western civilization overtaking them and the tribe integrates into the modern world and the culture no longer used then that tribe would be lost.
On the other hand if a tribe that has secumbed to western civalizeation but the descendents continue with the tribes culture, is it lost?