Your account has probably been hacked and used to send out spam, viruses or more phising mail. Contacting yahoo is the way to go because they probably locked it - possibly automatically when it started sending out mails by the thousands.
Also, if you stored any mails in there like password resets from places, account info etc, you need to change htat info as soon as possible. the spammers may or may not have actually read your mail, but once you gave them your password you gave them total access to everything stored in that mail box.
Spammers and scammers phising yahoo accounts is horribly common. I get at least one mail a day from scammers posing as yahoo. Yahoo is horrible with spam and scam mails. they are either incapable or unwilling to do anything about it.
If you are able to claim your mail back, you may want to consider migrating to a different address or service. Even after they can't use your mail to zombie out their junk, that address will be on their list and you'll be hammered with spam. I think my mom was getting a couple hundred spam mails a day after it happened to her.