Gmail – My Gmail password has been changed by someone

You could be in one of the following situations:

• you could have a keylogger/trojan on your computer;
• you could have logged in from a computer that had a keylogger/trojan installed;
• you could have been a phishing scam victim;
• you could have used a service that required your gmail account password (less likely) and they either store it in plain text and got hacked or were just rogue and sold/used it.

Now, you should:

• change all the passwords that you received on your gmail account, for each service/server;
• get in touch with any financial organization (e.g. banks) that might have sent you sensitive info on this email - if that's the case, follow bank account and credit card movements.

And last, you should switch to 2-step verification for your gmail account. Details are on gmail blog: http://googleblog.blogspot.com/2011/02/advanced-sign-in-security-for-your.html

Also, make sure your computer is clean - no viruses, trojans, keyloggers.

Edit:

It's also possible that you have the password stored in plain text and that file got somehow misplaced (trojan, file sharing error).

I'm very sorry to hear about your experience.

If you are able to access the account, I would strongly recommend going through the Gmail Security Checklist. Since the intruder was able to get in a second time, it's highly likely that they have access to one of your recovery methods (backup email address, answer to the security question, etc) or they have installed a keylogger on your computer. The security checklist will take you through all the steps to check all of these areas and re-secure your account.

However, you mentioned that you have cancelled the account, so I am guessing you are less interested in re-securing the account and more interested in recovering from the situation.

I recommend running full malware scans on your computer, and changing the passwords for any websites associated with your email address, especially if those websites ever emailed you your password in plain text. You might also consider whether pursuing legal action against the intruder is appropriate; depending on the laws where you live, you may have some recourse.