I've used WordPress for almost 15 years now, and I've never seen the frequency of random "Sorry, That Page Doesn't Exist" errors that I've seen in the last few weeks.

Clients have emailed in one-by-one, after the recent core upgrade to PHP 7.4. Well, at least that's the only correlation I can see between these errors.

I could be wrong on the cause, but I know the fix! And that's the most important thing.

It's definitely not as scary as the error itself. Trust me. It actually only takes a few seconds.

First, go to your WordPress Admin Dashboard and confirm your page(s) weren't deleted.

Then, go to Settings - Permalinks and click Save. That's right. Don't change anything, just click "Save."

Your pages should reappear upon refreshing. 

You're welcome. :)