Themes Admin page not loading

Hi,
I just upgraded to 3.0.5 today and also reverted back to a single site from a multi-site by updating my wp-config.php and htaccess files. Now I cannot get the widget Admin page to load. The widgets do show up on the front end of my website though. Does anyone have any ideas why that would happen and how to fix it? Nothing else seems to be wrong. Thanks.