It might be the issue mentioned in this post – if you have a page called “Achievements” – remove it, or call it something else.
http://wordpress.org/support/topic/page-not-found-85?replies=14
It fixed my website.
It sounds to me like clearing the cache would work for some of you. Here is what I did to solve the issue. Deactivated the last plugin I uploaded and cleared the cache. (Try loading the dashboard up in another browser, that you havent loaded the wordpress dashboard in before).
Shift + Reload. (but do get rid of your rough plugs first).
