Piwik stats remain flat

As the last version of Piwik would not run (provider using php lower than now required), I had to remove the new installed version and get back to the previous version. I have now acces again the the normal piwik graphic pages, but they remain flat (not daily update of the visitors).

Why is that? when looking into the admininistration of piwik, it stats that shell_exec should be enabled. How can I do it? Is it something which should be asked to my provider or can I do it myself, if so how?