Simply restarting your PC often fixes the problem.
If not, open Outlook Express and click on Tools > Accounts > Mail. Click on your Virgin account, to highlight it. Then click on Properties > Servers. Check that your POP3 server is shown as pop.virgin.net. (Your SMTP server should be smtp.virgin.net, but I'm assuming that's OK because you can still receive mail). Also check that your username is correctly shown. Delete and retype your asterisked password. Click 'Apply' and 'OK'.
If that doesn't work, delete your Virgin account. (Start as above, but click 'Remove' instead of 'Properties'). Then click Add > Mail, and create the account again from new.
Chris