All my mailboxes (Web Mail) have vanished and mail does not open on sidebar.

Hey bbbbak,

If we understand correctly, after encountering an issue using your email service providers webmail account you're now not able to open the Mail app on your Mac. If that's correct, try holding the Shift key while opening the Mail app to see if clearing the last saved state helps.

If not, what happens when you go back to webmail for the account? If you're still missing those folders, you'll want to reach out to your email service provider to work through that as this could be causing the issue with the Mail app on your Mac.

Let us know how it goes.

Take care!