Google Online Security Blog: Security warnings for suspected state-sponsored attacks: Today, we’re taking that a step further for a subset of our users, who we believe may be the target of state-sponsored attacks. You can see what this new warning looks like here:Google is rolling out a warning to a subset of its users whose accounts have been targeted by state-sponsored hackers. The link above goes over what to do if you're seeing the warning; only those who see a message about it when logging into Gmail or other Google services are thought to be affected. Even if you're not seeing it, it's a good time to invest in setting up two-factor authentication for Google accounts, which I highly recommend. It's minimally difficult to use once it's set up, and makes your Google account almost rock-solid against hacking attempts.
As always, if you're using the same password for other accounts, you need to change all of those passwords.