You do not have access to them. It is a poorly named clean up operation in Thunderbird. Read how it works here. https://support.mozilla.org/en-US/kb/compacting-folders

I think Airmail means the compact should not have affected your access to mail.

But if your mail isn't showing, then that is something you need to describe to us.

No, I mean it is a poorly thought out name. People think this is zipping up their files to save space and that they can be accessed later and that is not the case.

Berkshire - are you using the German locale or some other non-English version of Thunderbird?