What happens when I can no longer find authors of a paper that I am submitting?
This is a tricky one!  Since some venues may require each author to sign over copyright, it complicates matters above and beyond the ethical question of what is right.  

If you feel they deserve authorship, you should IMHO make a good faith effort to find them (web search, ask the university for contact info, etc).  But in the end, if you can't find someone and they contributed data but not writing the paper, I see no way you can include them as an author.  An acknowledgment would be appropriate.