Blog errors

One day, I logged into my admin page to write a blog post like I always do... and there it was: an unfamiliar message. 403 Forbidden - You don't have permission to access this resource. “...What is it? Did I do something wrong?” “Everything was fine until yesterday, why is this happening?” At first, I panicked. I hadn't installed any new plugins or changed my theme, my hosting was working fine with Cafe24, and I don't remember doing anything that could have caused the problem. But I couldn't access the admin page, my mind was blank, and my blog, Mystery Days, was locked...