It looks like my emails have been hacked, foul emails going out, no emails in folders. What do I do?

I opened my emails today, all OK. This afternoon, the email folders & site is completly empty. There is a profane sentence in the recent contact? area. Now I'm hearing I've sent old emails with profane additions as replies to some of my contacts. It certainly looks like I've been hacked. So what do I do now?