Can you provide a screen shot?

I suspect that you have lost the left hand panel that lists the accounts and their folders. This is switched on and off via the menu, IIRC, under View|Layout.

Press alt or F10 if you don't have a menu.

I have the mail boxes listed on the left as always. When I open the program, the preference page shows, which it never did before. Aside from that, I can only choose 1 of my mailboxes to open. I can not move from one mail box to another. For example, if I am working on a draft, i then need to save it back into the draft mailbox, close the program, open the program again and select a different mailbox to use. I can not be working in my "in" box, cut and paste info to add to a draft. I can not open my sent box to review something, then open my in box. Thankfully i still have all my emails / info, but each mailbox is now mutually exclusive for opening and working from. Emails do get sent.

Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Matt - You are my hero and new best friend ! I only had a couple of add-ons and disabled them, as well as the dictionaries. Maybe it was just re-booting the program, maybe it was the add-ons (I will not add on), but THANK YOU !!!!!! I have 30,000+ emails as I had been using Eudora for email since 1998 and Thunderbird is the only compatible evolution. My work depends on this program... Fingers crossed that was it and this will be a short term best friend relationship ! :-) Thank you again.

what add-ons?

oh yikes, I guess i should have written that down. neither were anything that seemed to have meaning to me. I also deleted cookies.

one would have been lightning... Thunderbirds calendar.