I use Ubuntu One on Windows, Ubuntu and Mac computers. I've not had any issue with UbuntuOne on the windows or Ubuntu machine and it has been file for months on the Mac until yesterday. Now it stalls at the File Sync In Progress and doesn't do anything. I have tried disconnecting and reconnecting.
more information is necessary to figure out what's going on - could you please contact support via https://one.ubuntu.com/help/contact/ ?
If the Ubuntu One Client is behaving strangely, disconnecting and reconnecting is not likely to help much - quitting the app (from the menubar icon to quit the background processes too) and restarting is more likely to help. Rebooting may also clear up any processes that have become unresponsive, if you can reboot.
If none of that helps, our support people can help you fix things.