I was using piwik 0.9 and observed that I lost one day of data from the interface (all numbers reported as 0). This was mentioned on the forum as well:
Here are the details: Piwik was properly recording visits, etc. and updating the interface throughout the day (I was viewing the current day's data).
At midnight, the traffic numbers for that day only, all went to zero.
In the piwik_visitor tables, all the data had been recorded properly, so I truncated all the archive tables and reloaded piwik with resolution.
I suspect that some problem occurred when piwik tried to write to the archive tables, failed for some reason, and then didn't properly update them later.
I had the live visitors plugin running at the time.
I would also like to add that I love piwik! What a great product and thanks to everyone who has contributed to it.
I think this is another symptom of #1458. What timezone did you set for your site?
This happened on the Piwik demo on Aug 6 and Aug 9. bug origin is still unknown at this stage.