Outlook Express has a very nasty habit of losing your account information. First, make sure that you know the following information;
1. Your username. (It's usually the bit of your email address before the @ symbol but some email providers require you to enter your full email address)
2. Your password.
3. The incoming (POP3) server address used by your email provider
4. The outgoing (SMTP) server address used by your email provider
You can find the POP3 and SMTP addresses from the support pages of your email providers website but, if your email is provided through a UK ISP, you can probably also get the information from here:
http://www.kitz.co.uk/isp/dns.htm
Open Outlook Express. Click on Tools>Accounts and then on the 'Mail' tab. If you've only got one email account it's probably already highlighted in blue. (If not, click on the account name to highlight it). Then click on 'Properties' and on the 'Servers' tab. Check to see that all of the information is correct. (I suggest deleting your password and re-entering it). If anything has to be changed, do so and then click on 'Apply' and 'OK'.
Even if everything appears to be OK, Outlook Express might still be using settings other than the ones which it is actually displaying. If so, start as above but, instead of clicking on 'Properties', click on 'Remove' and 'Yes'. (Don't worry. You're only deleting your account details. You won't lose any mail). Then click on 'Add>Mail' and work your way through the wizard to create your account again.
Chris