This issue most commonly is caused when you upload new files after you load the website. A session is created and the site is then replaced by another unix user or the website is run from a different user. As such the site is no longer allowed to reach your session information. A simple work around is to clear your session information; in Google Chrome, press F12, under the tab resources press Cookies, and then select and remove (del) any entries that link to your domain.
If this does not solve your issue, you best contact your hostingprovider as probably the session.save_path is configured incorrectly.