Aagh! The dreaded 'sticky message' problem!
There's an email on the server of your email provider which, for some unexplained technical reason, can't be downloaded to your PC. (It seems to happen quite often when a message has a very large attachment but sometimes there's no obvious cause at all). Since that message is the first one in the list of mails waiting to be sent, you can't get any of your mail.
Fortunately nearly all email providers allow you to access your mail via the web. Go to the relevant website (which will be your ISP's site if that's who provides your email service). Look for a link to 'email' or 'login'. Enter your username (which is usually the bit before the @ in your email address) and your password. You'll then be able to see what's in your inbox and, more importantly, to delete the item that's causing the problem. (It will probably be the first mail item in the list. If in doubt, simply copy any important mail content to, say, Word, and then delete everything). Thereafter you can revert to using Outlook Express and your mail will download normally.
Chris