WordPress not recognizing any Custom Post Types

All of a sudden, I can no longer access any of my custom post types from the front end. No matter the custom post type, I get a page error (not found) when I go to “View.” For the most part, I can access them from the admin area, and edit them, and they show up in search, etc., but if I go to the individual post (from the front-end feeds or the backend “View” choice from the editing windows) I get a Page Not Found.

This includes custom post types created by plugins, custom post types created by the Custom Post Type UI plugin, and hard-coded custom post types.

Even when I’m using the default theme and have only one custom-post-generating plugin at a time activated, I can’t access that plugin’s custom post type. These are plugins that have worked in the past with no trouble and which work fine on a clean install. I have not recently upgraded WP or added any new plugins. I am stumped.

Any ideas?