Folder in local folder inbox disappeared

I have been using Thunderbird for about 15 years. I use POP3 access to download a copy of each email into Thunderbird, and then use filters to move emails from each of multiple accounts into a subfolder under local folders. I have one account which is no longer active, but all the emails were still stored in the subfolder. Today, I went to look at the emails in this account, which hasn't been looked at for at least 5 years. When I clicked the subfolder to see the emails, I got a popup window which looked like an OAUTH access window, not the list of my stored emails. I'm afraid I didn't pay much attention to the popup window, I just closed it so I could try again. When I closed the window, the subfolder disappeared. I have no idea how to get it back, since I still want to keep those emails. Does anyone know how to get it back?