All my Local Folders have disappeared

In the menu bar, choose View > Folders > Folder Pane Header if not already ticked, to make that visible. Then, click on the '···' button to the right of the '+ New Message' button in that pane to display a list of options. One of the options available there near the bottom should be Hide Local Folders, which you have probably ticked, select it to untick it and make the Local Folders appear in the folder pane.

The following article shows the Folder Pane Header and explains the options available there:

https://support.mozilla.org/kb/new-thunderbird-115-supernova#w_look-and-feel-improvements-new-message-button-get-messages-folder-pane-options-meatball-menu

Done all that. Hide Local Folders was not ticked but still no Local Folders??

This happened when my PC did an update I think.

Ah! The following article may be relevant to you then, although I would expect other things to be missing if that was the case…

https://support.mozilla.org/kb/recover-user-data-missing-after-thunderbird-update

Yes. My Address Book is missing also.

Followed the link. No joy.

In TB go to Help > Troubleshooting Information > Profile Folder > Open Folder... then quit TB. In the profile folder open the "Mail" subfolder and make a screen shot of its contents. Do you have a backup copy of the profile folder?