Half my world is missing!

So I always make copies of my worlds so I can have them as a back up, but this time when I made the copy, half the world disappeared. I’m going to try to explain. The borders of the world closed in and blocked out half of the world but I can still see animals on the other side. I had built a house and now it’s gone and I had built a spawn home and I can only see half, the other half is up against the border. Help?! Btw the copy also had the same issue.