World not loading

Whenever I try to load one of my Minecraft worlds, it always never loads, and when it does finally after an hour or so, all you can see is the stars in the night sky. My Java and Minecraft are both up to date and windows is also up to date. It's only just started happening, and it took an hour to load the game. Other worlds load just fine, but this one doesn't. Also, when I WAS playing the world earlier today, I noticed a massive square hollowed out in the middle of my world (I'll attach a picture, sorry for bad quality) but you can't enter it, and neither can mobs. Plus, some of the structures aren't generating properly now. The seed for this world is 3490583591175743838. Thanks.