If you are using the UTM (Urchin Tracking Method) JavaScript for your Urchin stats, make sure that every page in your site has the one line of required JavaScript included. If pages do not have this line of JavaScript, or if you deleted the files "__utm.js" or "__utm.gif" from your "www" directory, Urchin will no longer be able to track sessions, visitors, or pageviews correctly.

Reference the UTM instructions here.

If you do not see any visitor or session data, and you have not setup the UTM system, reference the link above for instructions.