My email account was hacked, pls help me.

Hello,

Our business email account was hacked, and the hacker was sending emails to all our contacts asking money, i can't open our account anymore he might changed the password, or everytime i log in its was locked. What i can do? I asked help in yahoo already but still i'm not having the right answers. But until now the account is still active, i want to get it back because all our clients contacts was there and we haven't save any. We don't want he destroy our name for his personal interest. We want to stop him. Please help me! Thank you very much.