Welcome to Android Central! Who was this "someone"? How did they change your Google password? In order to change your Google password, this person would've had to enter your previous Google password first, so presumably it's someone you know well. Track them down and demand that they unlock the phone and tell you the new Google password. There's not much else you can do otherwise.
I agree with B. Diddy. And I would set up a stronger password. Avoid common phrases, anything found in the dictionary, sections of your date of birth, SSN, kids names, and pet names.