This behavior is most often caused by the Uplay servers undergoing maintenance, or a temporary outage. More often than not this issue will resolve itself with a little time. 

If you experience this behavior for an extended period of time ( an hour or more ) check the Uplay support site for any notices of outages.

If you see no notice, make sure the Uplay has been added to your trusted applications in any firewall or antivirus software you are currently using on your computer, in the case they are mistakenly blocking the connection to the Uplay service. 

Once you have done this, restart your router by unplugging it, waiting 10 -15 seconds, and then plugging the router back in. This will refresh your internet connection and may help to resolve the behavior 

For further assistance with Uplay, please contact Ubisoft support HERE.