Did you recently upgrade?
Please try these solutions and let us know if either work for you
1. Click Ctrl+A>right click the account and choose Edit>click the Online services tab and deactive the online services. Then (you may have to close and reopen QB) set up the online services again and try to access.
2. Set up a new bank account and try to download. If this works then you can merge this "new" account with the old account.
or
http://support.quickbooks.intuit.com/support/Articles/SLN42736
or
http://community.intuit.com/posts/cantr-import-bank-data?cid=e_replysubscription_v4