In rare cases, Piwik will show “no data” only for one day or one month, while other periods work fine and report visits as expected.
If you are not yet using at least Piwik 1.6, please update now (we fixed issues related to this problem in 1.6 and latest).

It is possible to re-process the specific days or months that are showing zero visit, check out this faq: Re-process reports.