[Plugin: WP Super Cache] Nothing being cached

I read some codes in wp-cache plugin when made a plugin recently. But reading codes is not a delighting work, I still donnot quite clear. So I only can give these advices as for your problem.

Have you modified wp-cache plugin before upgrade?
if not,
Did you install some new plugin after upgrade?
if not,
Try delete all files in wp-content/cache?
moreover, you probably have to delete *cache*.php in wp-contents folder.