Error 404 on my Dashboard

Whenever I click on Updates, Plugins, Settings, or try to update to WordPress 4.3 by clicking on “Please update now”, I am redirected to a blank page on my website that says “Error 404 – Not Found”. In addition, the items on the menu bar seem out of whack; sometimes when I scroll my mouse over them or select on them they shift up and overlap with other menu items and sometimes directly over top of each other like they’re on the same spot in the menu order, but as soon as I scroll my mouse over them they jump apart somewhat, though are still not evenly spaced. The website itself is not having any obvious problems and all the pages load fine. I can’t recall doing anything that would cause this. Any ideas?