Blog posts just not showing

I am thoroughly baffled by this one – the /blog page was displaying all blog posts, using index.php.

It’s now decided not to show any posts at all. It’s defaulting to archive.php but the posts aren’t showing on there either, only the sidebar widgets are. However if I click on a blog category, the posts are all there.

I have tried switching to default themes (same problem), and rolling back to earlier databases, but just cannot get it to work.

Any thoughts would be much appreciated.