An error occurred signing in Chat account

Hi friends,
When I signed in Chat account I got this error message, Offline alert you have been disconnected as someone has signed in with your ID on another computer. How will I sign in now? Please explain me how to change the password immediately. And also explain how to avoid this situation in the future. Help me please. Your help is great.

Offline Alert
You have been disconnected as someone has signed in with your ID on another computer.
Please note that if this was not intentional, someone may have stolen your password. Please change your password.
For further questions, please click here.
OK
