You've explained that your emails and folders have gone (I assume you mean that they no longer exist) but not how this came about. Please give some explanation of how you arrived at this situation.

Please provide screenshots of any errors associated with this that you are getting. Is this an IMAP or POP account? Are you running Thunderbird on a computer, Mobile, Tablet or something else (please specify)? Which version of Thunderbird are you using? Which Operating System are you using?