When a Microsoft Outlook account is set to "Work Offline" then this will prevent email from being sent or received. You can find this setting located in Send/Receive tab and then clicking on Work Offline. If you all of the sudden cannot send or receive email and you see that you are working offline, go to the same location and click Work Offline to turn it off and be able to work online again.
Typically working offline has been turned on in error.
To do this with the OWA Exchange - You can find this setting located by clicking on the Settings Gear in the upper right hand corner and clicking on Offline Settings.
Same situation as before - You can either turn it on or off in this location.