Further to Rojash's reply:
Outlook Express (and Outlook) have a nasty habit of losing the settings which enable it to connect to your ISP's (or other provider's) server. To be fair to Microsoft, it's not entirely the fault of their software. Antivirus programs need to temporarily change these settings, so that they can scan your mail. Sometimes Outlook Express and an antivirus program fail to communicate with each other properly and the settings become corrupt.
In Outlook Express (or Outlook) go to Tools > Accounts.
Click on the Mail tab.
If you've only got one email account it will probably already be highlighted. If not, click on the relevant account to highlight it.
Click 'Properties', then click the 'Servers' tab. Check that the POP3 and SMTP details match the ones provided by your ISP (or other email provider). All ISP's put the relevant details on the support pages of their websites but you can also find the relevant information (for the major UK ISPs) here:
http://www.idl.uk.com/email/isp_settings.htm
and/or here:
http://www.kitz.co.uk/isp/dns.htm
Check that your email address appears correctly.
Delete and re-enter your (asterisked) password.
Click 'Apply', then 'OK', then 'Close'.
Chris