If you found that anyone has hacked your Gmail account, You need Instant Gmail Tech Support
. First of all Check your Gmail last activity that from where your Gmail account has been opened with IP addresses, It shows your last 10 login details. http://www.gmailtechnicalsupportforum.com
Then immediately sign out your all other session. It make your account log out from another PC. After that change your password instantly and if your password already has been changed by the hacker you can recover your account by following forget password through your recovery number and mail id. Create strong password that no one can guess it.