I had a similar problem. After installing some plugin, I was not able to see the widgets page (it was blank). I tried everything, from disabling all plugins, to reinstalling wp (deleting wp-admin and wp-include), but didn’t work. Finally, I change the permalinks; I had it set in numeric form, and changed it back to default, and voila! It worked. (After that, I changed the permalinks back to numeric…)
I hope it works for you!