15,000+ emails have just vanished. Help!

I was working this morning when I noticed that my MacBook Air's Mail program said I had 417 emails, but only six showed in the list. I should have something in excess of 15,000 emails, going back several years but they have just vanished.

As far as I know, I didn't do anything to cause this. I didn't reboot the machine from when I had all the emails until now. I didn't empty the trash. I didn't move emails from one place to another. I checked the trash – not there. I rebuilt the mailbox. Still only six showing. I tried to find how to restore missing emails from Time Machine, which I use, but all I could find was a comment that said emails are not backed up by Time Machine. I have a hard drive connected to my MacBook with Time Machine running regularly for backups.

I contacted my ISP and they said Mail was notorious for strange and mysterious things happening to emails, which is why they never recommend using it. They said they could do a file restore, but it would cost $50, and might be up to a week out of date.

I can't believe that emails can just vanish with no way of restoring them.

Can anyone help?

I'm running a MacBook Air M2, 2022, OS 13.1.

Thank you.