Ubuntu One is Canonical's file and application synchronisation service. As well as file sync, U1 allows access to contacts, notes or bookmarks from other Ubuntu computers or through its web interface. It also includes the Ubuntu One Music Store and can stream purchased music to iPhone and Android devices. This service will be discontinued at the end of July 2014.
Be sure to check out the following links for additional Ubuntu One support:
Status page - There may be a service disruption causing the problem you're having.
FAQs - A list created and groomed by the Ubuntu One team of frequently asked questions that may provide the answer to the question you have.
Contact form - If your question is account related or you have specific issues with your content (particular files or contacts not syncing, etc) then contact the Ubuntu One team directly for support.