In most cases, Outlook and your outgoing mail server are unable to communicate, resulting in the email being stuck in your Outbox. If your outlook won't send emails here are a few things you can try out :
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Internet service providers often limit ports that may be used by outgoing / SMTP mail servers, so verify with your provider to make sure the port number you're using isn't blocked.
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To be certain that your mail server settings are current, contact your email address provider. The outgoing mail server settings for your Outlook account may no longer operate if the port numbers or authentication methods were changed by the service provider. This might happen even though your Outlook account used to send emails without issue.
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Verify with your email service provider to see if you've gone over your daily or hourly email sending limits (or other send limits enforced by your email provider)
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You should check your Outbox emails to make sure they don't have very big attachments (10 – 20 Mb or more) that you don't want to send. A connection to the server may be lost before Outlook can send a big email; this could happen.
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Use Google to search for the exact error code that you got when you tried to send an email from Outlook. It's likely that this will lead you to a specific solution. It's also a good idea to look at this list of the most common send and receive error messages and how to fix them.