The new version of ArcGIS Online was released last week (3rd July 2013). A handful of users have come across problems when trying to login to their Organization account (for example an error stating "Organization Not Found", or user roles appearing blank once logged in).
These issues appear to be related to cached website pages and cookies being stored, referencing the old arcgis.com information. To resolve the issue, please clear the browser cache by doing the following (depending on the browser you are using):
Tools > Internet Options > Browsing History > Delete
Ensure that "Temporary Internet Files" & "Cookies" are selected, and ensure "Preserve Favourites website data" is unchecked
Customize and Control Google Chrome > History > Clear browsing data...
History > Clear Recent History
If you continue to receive such errors after trying the above, please contact Esri UK Technical Support