We upgraded to 2.6 and everything worked fine for three days until last evening. Then I logged on to the site and saw all textual content and links but no template...and the wp-admin "Design" area shows no templates nor do they come up in the preview.
+ Deactivated all plugins. Nothing.
+ Cleared all caches. Nothing.
+ Removed any traces of "WP Super Cache", including those in wp-config.php. Nothing.
So now we're going to downgrade to 2.5.1 which I do NOT want to do so thought I'd ask you all first. Any ideas?