So, I have this issue, where this happened two times. There is two pages - blog (which is based in index.php) and Thank you page (page-thanks.php).
In some conditions, these two are blank and in "admin bar" (black one on top of the page) is showing just "Blog" title and can't edit the page.
On the page, there is just header and footer (it's loading correctly). But where should be content, nothing is loading.
Query Monitor is showing in "Template" hooks just "home.php" and "index.php" as final load file.
First time, I switched off WP Rocket and it started working. The second time, I activated twentytwenty theme and then reactivated my theme.
And in both cases, it stared working as it should. But there is no log of any kind about error, warning or anything. I can't reproduce it anywhere and it happened only on PROD server - not on localhost.
I have no idea, what to do or how to investigate that.
PS: I am developing sites for years and this is first time, I see something like that.