My account have been stolen

I did not log in for about 3-4 days because of working,and today I cannot log in with my username and password.Think some one stole my account and changed my password during these days.I do not know how can he know my account information(the plugin may be the problem).Now,I can not log in or change my password because he logged in the account rencently.
So,I wanna ask what should I do to get back my account?
I will appreciate it if you can help me,thanks.
So,I wanna ask what should I do to get back my account?
I will appreciate it if you can help me,thanks.