Sorting from A-Z Suddenly Not Working

After cell 566, my spreadsheet stopped automatically formatting. I highlighted a bunch of cells underneath #566 and finally got it to format uniformly, but now it won't sort numbers 567-569 from A-Z in the "C" column. It appears that the program thinks anything underneath #566 is a new spreadsheet.

I've Googled and tried a bunch of stuff. It's maddening. I do not want to accidentally delete this.

I also want to thank the mods here for having the option to post images when you actually can't. I had to rewrite my entire post, which has taken up more time.