Which Operating System? Which version of Thunderbird? Are you running an IMAP Account or POP? Have you checked the sizes of the databases in your Profile?

Windows Version 128.5.2esr POP-account I do not know how to check the sizes of the databases... (Am an old woman who is not very good at computers)

IF the index is not in sync with the actual emails stored in mbox file then it may not be able to access the stored email. So if emails are still available then a folder repair should sort out that issue.

• Right click on Folder and select 'Properties'
• click on 'Repair Folder'
• Click on OK

If you have an Anti-Virus and it's allowed to scan profile files and folders and it's allowed to try and Fix anything then it's possible th Anti-Virus has corrupted the file storing the emails. You see, Anti-virus products do not understand that a single text file stores alot of emails not just one bad one. Advise is to make your Thunderbird profile folders exempt from scanning OR at the very least never allow any Anti-Virus product to auto or manual fix anything it may detect in a Thunderbird profile. Anti-Virus would not effect an index file which can explain why it thinks something is there when it is not.