Can't Save/Publish new Posts

When I try to publish or save a new post on my blog, I get redirected to the list of published posts, and there is no trace of the draft of my new blog post. When on the “Add a new post” page, I see this warning: “Warning: Creating default object from empty value in /homepages/22/d343542997/htdocs/wp-admin/includes/post.php on line 569”

The dashboard of my new post page has changed as well, before I could select tags for new posts from a menu and that’s no longer there. I was also able to publish directly before and now there is a button to “submit for review”. I didn’t make any of these changes- I have no idea what happened. Please help!