If a single conversation disappears after a keyboard action, check whether it was marked ignored. Thunderbird's Ignore Thread feature is designed to keep a mailing-list conversation out of the normal view. It does not delete the messages, but later replies in that conversation do not produce the usual new-mail notification.

Identify the conversation before changing its state

Note a known subject, sender and approximate date. Select the folder where the conversation belongs. From the View menu's Threads options, enable the display of Ignored Threads. This exposes ignored conversations for inspection without deciding that all of them should become active again.

Find the expected conversation and compare a known message's details. Thunderbird documents K as the Ignore Thread shortcut, so an unintended key press while the message list had focus may explain the change. Treat that as a possible history, not proof of what happened on another person's computer.

The related Ignore Subthread action applies from a selected reply onward. This matters when the beginning of a discussion remains visible but one branch of replies is being skipped. Inspect the relevant branch rather than assuming the whole mailbox has a delivery fault.

Restore only the conversation you need

For a thread that should return to normal use, select its message and toggle Ignore Thread off through the context menu. Leave unrelated intentionally ignored discussions alone. Then restore the preferred view setting and revisit the folder to check that the intended conversation remains visible.

Use an existing known reply for this visibility check. There is no need to send a new message just to prove that the old one was hidden. If future notification behaviour matters, observe the next ordinary expected reply and record its result separately.

If the conversation is absent even when ignored threads are shown, continue with folder location and search checks. Repeatedly toggling ignore settings cannot establish where a genuinely missing message went. The diagnosis here is complete only when the known conversation can be tied to its ignored state and deliberately restored to the normal view.

Source: Thunderbird documentation.