A fairly common problem with Microsoft Outlook is that it goes into offline mode. This mode is for using Outlook when you have a potentially insecure Internet connection (at an Airport for example), and it prevents Outlook from sending and receiving messages. Look at the bottom of the Outlook window for a status message that says, "Working Offline".  If you see that and your Internet connection is trustworthy (at your office or home for example), click Send Receive at the top of the Outlook window, and then click the "Work Offline" button to toggle that mode off.