Edit Post – 404 Page Not Found!

Hello! Bit of an alarming situation.

I’m not sure if this is due to the new version of WordPress, or if I somehow accidentally did something, but whenever I go to edit a post or a page, I receive a “404 Page Not Found” error. I tried disabling the plugins and theme I’m using, and going to edit again, but the same problem occurred. Not quite sure what to do here.

I’d greatly appreciate any advice. Bit of a newbie to this, and would hate to lose all the work I’ve put into this thing over the past few weeks.