You need to log onto your BT email account via a web browser and check that your emails and folders are all there.
If the folders and their contents are still on the BT servers it sounds like when you re-set up your email app you set it up as a POP3 account.
A POP3 account does not replicate the folders on the server. You would need to set your app up as an IMAP account which does replicate the servers folders.
A quick way of checking would be to send an email to your self. If you have your email set up as an IMAP account the sent email will show up in your sent folder on your computer and the sent folder of your phone.
