First make sure that you know your username (with your email provider), your password and the POP3 and SMTP server addresses for your email service.
Don't go into panic mode just because that sounds a bit technical. Those addresses will just be something like pop3.btconnect.com and smtp.btconnect.com. If you get your mail through one of the UK's major ISPs, the information you require will probably be here:
http://www.idl.uk.com/email/isp_settings.htm
(If not, they'll be on the support pages for your ISP, or other email service, as appropriate. Or you can phone your email provider - which is probably your ISP - and ask them).
You should also note whether your server requires authentication. (Unless that link specifically shows 'requires authentication', or your ISP tells you so, you can assume that authentication is
not required)
Then click through each of these things, in the order that I've given them:
Start > All Programs > Windows Mail > Tools > Accounts
If you've only got one email account, it will probably already be highlighted. Otherwise click on the name of the account to highlight it.
Select 'Properties'.
Click the 'Servers' tab and check that the POP3 and SMTP addresses are correctly entered. (At least one is probably incorrect. Correct any errors). Check that your account name is shown correctly. (Correct if necessary). Re-type your (asterisked out) password. Unless you've been told that your server requires authentication, there should be no tick in the relevant box. Click 'Apply', then 'OK'
That will probably fix the problem. If not, start as above but (instead of clicking 'Properties'), click 'Remove' (and, when warned that you're about to delete the account, agree to it). Then click 'New' and set up your account again. (It's dead easy.