If you can somehow regain access to it (e.g. by using the forgot-password question or your alternative email), login, change the password and if you are re-using that specific password, change it everywhere else. If your account was an email account, you need to think about all accounts that connect to it and could potentially be already compromised, and change the passwords there as well...
In other words, if someone hacks your Gmail, and you have facebook.com, twitter.com connected to it, you are best of changing all your passwords.
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