I think the title is self-explanatory, there is no data when I check Users tab (same result for different websites and time periods). I haven't checked users tab for while, but the issue is probably connected to the fact that i just updated piwik to the latest release (2.17.1-rc1).
All the other features like visitor log, actions and goals work just fine. The diagnostics only show problem with "LOAD DATA INFILE", but as far as I know this shouldn't cause the issue.
Are you tracking users? http://piwik.org/docs/user-id/