That's bad news. It was a popup? they often prompt fraudulently for a password, to get you to divulge news or info that is private to you and vital to your account.
This is called a 'phishing' scam.
Can you still access your GMail account?
If so, change its password ***** something you can remember and type in for sure.
Then log out of your GMail account, restart your computer, log back in.
See if that works, firstly (if your GMail account password ***** been changed already - fraudulently for sure if so, given what you have said - then it won't change again for you, I doubt.)
It is necessary to secure the security and continued access of your account ... that popup was NOT normal, if it happened on a computer rather than a handheld or tablet device. So your account is in some hazard at present.
Let me know how it goes, please.