If someone has hacked your Yahoo and changed the password, you're screwed. No two ways about it, they humped you, and how.
The problem is that they must have brute-forced their way in, meaning no matter how hard you try, you'll never get that data back. If Yahoo won't help, you've got to keep asking them as they're the only ones who can fix the problem. If they absolutely cannot help, you've got to restore all your info from your local backups.
You do have local backups, right?