There are a number of reasons on why your messages are not getting send while you're using MS Outlook.
The most likely reason, however, is that your Internet Service Provider (ISP) has modified their outgoing mail server settings for your area.
If this is the case, you will need to contact your ISP directly, and request that they provide you with their new outgoing mail server details. You will then have to take these details and use it to modify your Outlook settings, specifically the field labeled SMTP Outgoing Mail Server.
If you need more information, click here for a list of error codes, as well as other MS Outlook troubleshooting and repair advice.
If you are receiving the Outlook Not Responding error message, you might also want to consult this article about the topic.