Seeing a screenshot may be helpful. Without that, check that view>headers is 'normal' and that view.message body as is 'original HTML'. If you do not have the menu bar set, click the alt key so that the view options are visible.

The body of all of today’s emails are shown in computer code, not readable. Yesterday’s messages are fine - if they have already been opened they are readable. We have restarted the laptop and Thunderbird several times without success. Nothing in settings seems to apply and no settings were changed. Some now coming in okay. Brief sample of what was received- all are much longer.

<font color="3D">Breaking and Entering= </font>

it seems that HTML is not being properly processed. I suggest - exit thunderbird - go to thunderbird.net and download a fresh copy - install the download over the current installation -

WAIT! before doing that, highlight the inbox, rightclick, select properties, and then click 'repair' = that often works