You've not given us a lot of information.
Firstly, I'll I assume that you're using an email client (such as Outlook Express, Outlook or Thunderbird), rather than a web-based service.
Then I'll guess that the most likely client is Outlook Express (simply because that's what on most people's home PCs).
Outlook Express (and probably Outlook as well) has a nasty habit of 'screwing up' your account data which it needs to send and receive email messages. It's happened to me several times (including twice this month). It's annoying but really easy to fix. Or rather, it would be, if I knew the settings required by your particular ISP.
Please post again, stating which ISP you're using, and I'll try to guide you to a possible solution.
First, however, please press Ctrl+Alt+Delete. Enter the Task Manager and disable everything associated with Norton. (The chances of any harm coming to your PC if you run it without a firewall for a few minutes are effectively nil) Then try sending yourself an email. If it gets through, you'll know that the problem is with Norton (and everything I've written above is completely irrelevant!). If the mail still doesn't get through, I'll try to tell you how to fix your Outlook Express settings.
(After your test email, simply restart your PC to switch your firewall back on).
Chris



