I'm using a static home page, and a secondary page for posts using default template (index.php) ("Default Template" selected on the Edit Page page). Everything worked fine this morning.
This afternoon the posts page no longer uses index.php to display posts. I don't know what template it's using, but it's not index.php (I've modified it as a test with no result). No matter what template I try (archives, links, etc.), nothing changes.
Any thoughts on what happened? How do I figure out what template it's using?