One of the most useful icons in MS-Outlook is the network connection icon on the status bar at the bottom-right of the window. If it says "Not connected" (or similar) when hovering over the icon and there is unsent e-mail in the outbox then the mail transport layer is having issues.
Has your windows password expired? Was your Domain Controller rebooted (or IP address changed)? Is your DNS working? Microsoft use all sorts of cache to make their systems appear fast, unfortunately it also can cause weird problems.