This is the error I received after updating a WooThemes theme. I reverted back to an older version of the theme and was then able to log into my wp-admin again. However, now in my Incoming Links dashboard widget I see the same here:
Fatal error: Class 'Walker' not found in /home/scottwydenvps/scottwyden.com/wp-includes/post-template.php on line 965
I did not change the WordPress install at all. Any ideas what happened or how to fix it?