Permissions mysteriously changed

As a former user of Nucleus, I installed WordPress on one of my sites yesterday. I also purchased and installed a third-party theme. During the entire session, everything worked well. I didn’t yet change any settings, because I was just looking around to familiarize myself with WordPress. I created one post as a sample and uploaded a couple of photos. All of this was fine. Today, when I access the wordpress directory, all files, whether the index file or the wp-login file or any others, show a permissions error: “Sorry, but you don’t have the administrative privileges needed to do this.” There is nothing on the page, so I have no way of actually logging into WordPress to take a look at the permissions. Any idea how I can resolve this?