we had a similar question recently: 
Why am I getting spam from myself in Yahoo Mail?

There isn’t much you can do. However, if you want specific advice on what you can do, I suggest to ask a new question.

first of all, change the password to your Yahoo account, as well as the secret question and answer that secures your password.

Depending on how you access your Yahoo Mail, it could be that your account was not hacked. You could also be dealing with malware!

Where do you store your contacts? Online or on your computer?

Did you scan your computer for viruses, trojans, worms, and adware? For advice on which security tools to use, check out the answers to the following questions:
http://www.makeuseof.com/answers/security-programs
http://www.makeuseof.com/answers/recommend-free-virus-protection-program

You may also be the victim of a phishing attack. In that case someone is using your eMail address to send mails, which is very easy to do. The eMail headers reveal what computer the mails are really coming from.

Apart from that, the following article may be helpful for future hacking attacks:
http://www.makeuseof.com/tag/are-you-sure-your-email-isnt-being-hacked