Have been using Thunderbird for some time as my main e-mail but now my files have disappeared from there into BT how do I get them back into Thunderbird please.
It's possible that you've got Thunderbird configured to use the POP3 protocol, rather than IMAP. Go into your account settings to see which you're using. This might then become relevant:
https://support.mozilla.org/en-US/kb/switch-pop-imap-account