Unity "Files & Folders" lens can't find anything
Solution 1:
The problem could be in corrupted FTS index. See the bug report.
The solution is to remove the index - this shouldn't affect your overall history of activities.
zeitgeist-daemon --quit
rm -rvf ~/.local/share/zeitgeist/fts.index/
zeitgeist-daemon --replace
(Based on Michal Hruby's solution from the linked bug report.)