Microsoft Outlook client can occasionally go to Working Offline mode. You can detect it from the status message located in the bottom right corner of the client window:

 

You can switch the mode back to online through Send/Receive tab and clicking Work Offline icon from the toolbar:

 

Status message located in the bottom right corner of the client window changes back to Connected after a while:

 

If Working offline mode persists after performing these steps, make sure your computer has connection to internet and restart your computer. If issue is still persisting, contact Digital services.