Outlook Express has a nasty habit of 'losing' your account details.
First, make sure you know the POP3 amd SMTP server addresses for your ISP (or other email provider, if appropriate). They'll be listed on the appropriate website or, if you're with one of the UK's main ISPs, you should be able to find the details listed here:
http://www.idl.uk.com/email/isp_settings.htm
and/or here:
http://www.kitz.co.uk/isp/dns.htm
Then open Outlook Express.
Go to Tools > Accounts > Mail.
If you've only got one email account, it should already be highlighted. Otherwise click on the account name to highlight it.
Click 'Properties' and the 'Servers'.
Check that the POP3 and SMTP addresses are correct. (If not, change them).
Check (and change, if necessary) the account name.
Delete and retype your (asterisked) password.
Click 'Apply', 'OK' and 'Close'.
If that doesn't work, try completely deleting your account and then creating it again. To do so, follow the instructions above but, instead of clicking on 'Properties', choose 'Remove' instead. Once you've deleted the account, select Add > Mail and set up your account by following the instructions in the wizard.
Chris