If this is an IMAP account, right-click the account in the Folder Pane, Subscribe..., Refresh, and see if the missing folders can be checked (subscribed). If it's POP, the folders are probably still in the current or previous profile, but hidden (but recoverable).

It was POP. How do I find them? Thanks.

If the account was added again, its mail would be located in a new subfolder of the profile, e.g. in Mail/pop.gmail-1.com, whereas it was previously located in Mail/pop.gmail.com. Help/More Troubleshooting, Profile Folder, Open Folder, to open it in Explorer. To make TB read the previous folder, Browse.. the Local Directory in Account Settings/Server Settings to the previous location.

If the account wasn't re-added, reply here for further info.

TB was uninstalled. The new installation has captured my address book but none of the folders.

and it's not a gmail account.

The gmail folder was just an example, but TB creates folders with -1, -2,... appended to the server name each time the same account is added.

Were the folders stored under the account or in Local Folders? Close TB, open the Mail/<popservername> and Mail/Local Folders locations and look for mbox files (large files with no extension) named after the folders. If you see the missing folders, delete the .msf files that have the same name as the mbox files, then restart TB.

I've found two iterations: The current at ...Mail/mail.optonline.net/ and what I suspect is the original at ...Mail\pop.optonline.net/ with a list of subfolders. Can I meld the two?

Close TB, copy the mbox files, renaming them if necessary, from the pop.... folder to the mail.... folder, restart TB and see if they appear in the Folder Pane.