Yahoo has a procedure for recovering lost passwords, but of course, it is not perfect.
In order for it to work, you have to have given Yahoo an alternate valid e-mail address when you initially set up your account; in which case, upon your report of a lost password, they will e-mail you a new random password.
If your account was really "hacked", then the person who did it can change your alternate e-mail address in your profile to one of his own, in which case, you won't be able to get that account back.
It is probably just a case of Yahoo having issues though, like Peale said.