Product Categories disappeared!

Over the weekend, my products have decided to no longer be categorized. On my backend, it says there are no product categories, however it shows that there are 2 empty pages of product categories. Whenever I try and search for the category, it says there are product categories found, but they are not showing up.

I tried to create the same categories as before, but it won't allow me because the url is already taken.

Whenever I go into the single product edit page, I try and check off the category my product is under, but it is only showing "most used," therefore, I cannot link all my products to their designated category.

I just need to figure out three things:

1– How to get my product categories back? (on front and back end)
2– Why and how did this happen?
3– How can I prevent this from happening again?