"You do not have permission to…"

I updated to the latest WP and was trying to change my layout when everything started acting really odd. First, the layout and sidebar refused to work properly, but I got those figured out sort of. Now, it’s just whenever I try to log in, it takes forever; when I press login, it just goes back to the blank login screen, doesn’t say wrong password/username or anything. Sometimes it starts working if I clear everything and restart my browser, but most times I have to type in like /wp-admin/themes.php or some other link to get it to start working.

My biggest issue is that it is no longer allowing me to preview my posts, saying I do not have permission? It’s my site, how can I not have permission to preview posts?

Did I do something wrong, or is WP messed up? Thank you!