Have you set up or accessed your email from another PC/device i.e.. Another computer or a Smartphone? If so, the device may have pulled the emails off the server (rather than leaving a copy behind so other devices can access the same email). See following link http://btbusiness.custhelp.com/app/answers/detail/a_id/7290
Check your Webmail (by logging in via https://myoffice.bt.com/login.aspx ) to see if the mail is still there or not (Remember to check the deleted items folder + recover deleted items if you have the option available.
Is your email server setup type pop3 on Outlook Express? (Check by clicking Tools, then selecting Accounts, ensure the Mail tab is selected, click to view properties of your mail account and look on the Servers tab).
Can you remember recently getting an email from BT Business regarding important changes to your email service? (If so, your mail has been moved on to a new server and your Outlook Express settings will need updating, to remedy this - again log in to Webmail to find a link for "Download Configuration Tool" - this will do the work for you).
Last of all, if it's non of the above, try using the find function to search your PC for the email (you may have accidentally moved it or unknowingly changed a view setting of your mail client.
Let me know how you get on!
