Do you have a folder pane showing? On Menu Bar (If menu bar hidden -press 'Alt') 'View' - 'Layout' - (Classic view? your choice) - tick 'Folder pane'

Or is your folder pane showing, but some folders are missing? Give us a screen shot.

How do I create a screenshot of my problem?
https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem

TB-38.3 Win10-PC

Thanks so much for your reply..... the first fix didn't work. I do have the folder pane showing but half of my folders are missing-- here is a screen shot. Thanks again, Daniel.

Have you set your accounts to use a 'Global Inbox', the 'Inbox' in 'Local Folders'? When you check Tools - Account Settings' do you see your missing accounts listed above 'Local Folders' in the left window? (first image) If not, then the following does not apply.

Check the settings in each account listed, account name - Server Settings in the 'Message Storage' settings area, press Advanced you should see your choice of where that account's 'Inbox' is located. (second image) If you selected 'Inbox for different account', then you will not have an entry for the inbox in Thunderbird's main Folder Tree.

If this is what happened, you have not lost anything, except the clutter of duplicated folders, and the hassle of accessing each one. Messages from all of your accounts are combined into a single folder tree in 'Local Folders'.

It can be disconcerting. It looks like something went missing, but it actually makes things easier for you.

If you really do not like the change, you can change it back, but messages already in the 'local folders' will not be automatically returned to the specific account folder of the same name. You will have to change the location of where you save messages in several sections of each account, 'Server settings', 'Composition and addressing' and 'Junk Settings'

Does this help?

TB-38.3 Win10-PC