May be you accidentally activated the item "Show only messages from people in your address book" in Thunderbird's Quick Filter (see image)?

Thanks - but the quick filters might need some study. What you call the Address Book actually says "Contact" with a little blue toggle light. This seems to be saying "Contact" or "Contact", further defined by the little toggle light. But the toggle doesn't correlate with the text (which doesn't change with the state). Perhaps I need to study this more. Regardless, it's a continuous challenge! The Unread button, for example, always shows the same messages, either "Unread" or "Unread" regardless of the state. 24 hours later I can't recall which state I'm seeing - either "Unread" or was it "Unread"? It used to be in the old days that unread mail was bold type, and read mail was normal type. The rest were just buttons that could be ignored. Simple. Idiot-proof. I think I'd prefer how it was in the old days.

Here is a new screen shot with the five Quick Filter buttons and their names (not the tooltip as in my first image). In my case the button "Contact" is activated

Yes, all well and good. Big thanks so far. But what is that telling me? It says "Contact - activated", but how do I know what that means? It's "contact" with a blue toggle light. So-o-o-o-o . . . ?? Where's my emails?

Did you see the tooltip n my first screenshot explaining that the button "Contact" of the Quick Filter toolbar can be used to display only messages from persons which figure in your address book.

Or another situation: the Inbox message count indicates that Inbox contains 3 unread messages, but scrolling up and down hundreds of messages in thread pane you are unable to find those unread. So you click on the "Unread" filter button and Thunderbird will displays only the 3 unread messages in the thread pane.

Nobody forces you to use the Quick Filter toolbar and sometimes people get caught in a trap: clicking by accident on one of the Quick Filter buttons they don't understand what happened and why they can no longer read their messages.

1. I don't understand where you are getting the wording "Contact" - everything in my UI has wording "address book". A screen shot would help. (But that's also not relevant to your problem)

2. A good test here, since other people are not seeing what you describe in terms of unread messages, is do Help > Troubleshoot Mode. It will change the appearance of Thunderbird - ignore it. What matters, is does the problem still reproduce.

3. If it still reproduces, then right+click on the folder, do properties > repair.