First, let's touch on the basics. Go into VIEW - LAYOUT and make certain your Folder Pane and Message Pane are checked. Let me know one way or the other. If nothing changes, do be specific about what is selected in this option and how it appears on your screen. Remember, big brother may be watching but I'm not :) .

many thanks Smokva for your kind and speedy response to my panic. Whilst waiting i went into my add ons as others seem to have done and disabled everything. then restarted Thunderbird. this seemed to have worked. I then went back and put the add ons back on with no issue - sorted.

thank you once again - great to know you're there.

You're most welcome. Glad things are back to normal.

Thank you for letting us all know it worked out.