Tell us more on what you think happened with them. Is this an IMAP account? Were the messages in one of the account folders, or were they in a Local Folders folder?

Thanks David for your response, apologies for late reply. Don't think they are an IMAP account. No is the answer to your other questions. They were just ordinary mozilla thunderbird emails with attachments.

Is the folder still there, or is it also missing? If the messages were in a folder in Local Folders, they may still be on your PC, but knowing in which folder will be useful for you. Were they deleted or were that messages that you had been keeping and had not accessed lately? Is everything else working OK, such as the email account and other messages? Until I know more information, I cannot suggest where to look.