What type of email account is this, POP or IMAP?

If POP, check under Tools > Account settings > Server Settings to see if there are any options to delete / remove email automatically from the server.

If IMAP, your email provider may have restrictions on email storage that automatically delete older email. Ask your provider about this.

I did as suggested and still lost emails last night. Any more ideas?

It would help if you shared the information you find. Can you take a screenshot of your settings? If you need help to create a screenshot, please see How do I create a screenshot of my problem?

Once you've done this, attach the saved screenshot file to your forum post by clicking the Browse... button below the Post your reply box. This will help us to visualize the problem.

Is this email account configured somewhere else (mobile device, laptop, desktop, friend's / relatives place, etc.)? The email may be deleted by filter or other conditions at one or several of those places. If you change the password they will all stop working, and you can troubleshoot the problem by replacing the configured password with the new one, one place at a time, until you check all locations settings.

At this point this is not specifically related to Thunderbird, but it would still help to ptovide a screenshot of your settings to double-check that.