Check to make sure work offline is not selected. This can be found in Outlook, on the Send and Receive tab - make sure work offline is not selected, if it is, click on it to turn it off. Also make sure you are connected to the internet. Microsoft offers some troubleshooting tips on this topic at: http://support.office.com/en-us/article/Switch-from-working-offline-to-online-2460e4a8-16c7-47fc-b204-b1549275aac9.