Has my email account been hacked? Please help.

This morning I received (3) "mail delivery failed..." and a response from CNN about a question that was sent from my email addressed. I looked at the reasons why and to who the (3) emails were sent back and I never sent them. Has someone able to hack into my email account and used it to send spam messages? For all I know hundreds of email mails may have been sent out using my my email address. How to stop this and prevent it from happening again?