Due to the nature of changes in Page collections that were introduced in Hugo v.0.57.0 now a large portion of themes in the Hugo Themes Showcase feature an empty homepage and/or post lists that point to section lists (instead of individual posts).
We are aware of the issue and we track it in gohugoio/hugoThemes/issues/682
If you are a theme author please visit the above link for all the specifics and also check whether your theme is on the list provided. There are instructions about fixing the issue, so if you can please look into it.
Also in said issue we will discuss about actions that will need to be taken if these themes are not fixed anytime soon.
So if you want to voice your opinion please do so on GitHub.