Check to make sure that Outlook is not in Work Offline mode: Along the top of your window, you will see multiple tabs; choose the one that says SEND/ RECEIVE.
If it is offline it will look like this:
Click it to work online and it will look like this:
Sometimes Outlook will switch to this mode when your computer momentarily loses connection to the Internet (switching to wireless, power outages, etc.) Turning it off will make your email send and receive regularly again.