I got the same problem.
Tried to fix this in different ways, like restoring from backup with no success.
I disabled all my plugins and Widget pages came back :-) And narrowed it down to Google Analyticator.....
I found that by resetting the plugin to it's initial state, I could have a active Google Analyticator plugin and my Widget area worked kept working :-) You'll find the reset link on your plugin page left to the "Deactivate" link of the plugin
I re-entered my UA tracking code manual and things did works again :-)
My best guess is that the latest WP 3.5 update and this plugin didn't go well together and gave some java script conflicts, that can be resolved by resetting the plugin.
I finally deiced to switch to "Google Analyticator" cause I use it on all my other websites and like to get things more consistent. So no hard feelings, these things can happen.
Hope the reset tip will help others with this problem to regain all their WordPress functionality