What email client are you using?

What email application are you using?
What internet connection do you have?
Who is your email service provider?

Try this:
**Delete all the emails in your outbox. Try send again.
**Your anti virus/firewall could be interfering. If you are scared to play with the AV/Firewall's settings then uninstall your anti virus and try again. Reinstall it after you are done.
**Phone your internet provider and ask if there are any problems.