If you load up your blog and see something like this:
Database error: [You have an error in your SQL syntax. Check the manual that corresponds to your MySQL server version for the right syntax to use near ',,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,]
SELECT DISTINCT ID, category_id, cat_name, category_nicename, category_description, category_parent FROM blog_categories, blog_post2cat, blog_posts WHERE category_id = cat_ID AND post_id = ID AND post_id IN (,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,)
Warning: Invalid argument supplied for foreach() in /path/to/wordpress/wp-blog-header.php on line 478
Don’t worry, nothing happened to your data or WordPress. Your host probably just upgraded PHP to version 4.3.10 and forgot to upgrade the Zend Optimizer. We already have a support thread about it. The PHP download page says:
Note: Due to an incompatibility between earlier versions of Zend Optimizer and PHP 4.3.10, it’s recommended to upgrade to the latest version.
So contact your host and let them know what’s happening, and they should be able to fix everything.