Every once in a while, strange permissions issues crop up. You may be able to view content just fine when logged in, but anonymous users cannot view content.
If you've double-checked that permissions are correct at admin/user/access, and the problem persists, go to admin/content/node-settings (the link at admin/content may say "Post Settings").
There, you'll see a section that says,
"Node access status
"If the site is experiencing problems with permissions to content, you
may have to rebuild the permissions cache. Possible causes for
permission problems are disabling modules or configuration changes to
permissions. Rebuilding will remove all privileges to posts, and replace
them with permissions based on the current modules and settings.
"Rebuilding may take some time if there is a lot of content or complex
permission settings. After rebuilding has completed posts will
automatically use the new permissions."
Plus a little button that says, "Rebuild permissions"
Click the button.
When it asks if you're sure, click the button again.