Login cookies and sessions can sometimes become corrupted, it's best to try deleting them via your browser privacy options. In Firefox, on the Toolbar it's Tools > Clear Recent History, and tick the Cookies option and click Clear Now.
In IE, Tools > Internet Options > General (first tab) | Browsing History "Delete..." button, and then tick Cookies and Delete. (If in IE you cannot view the Menu Toolbar, click Alt to see it.)
Hope that helps.
