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What message, exactly, shows on the phone, when you try to use it?
What phone is it, by maker and model ?
OK. I get the picture... just a moment...
Gathering info and composing an answer... just a moment please...
You must log on to Google Device Manager, through your Google account on a different device than the phone - a computer for instance, or even a tablet.
In Google Device Manager, change the password ***** your device's unlock code (not the Google account password! not! repeat not!), to something simple, e.g.
and then see if (after waiting for at least 30 minutes, to allow servers to all propagate the new password ***** their systems and off into your phone as well) entering that password
will unlock your device.
A. The Google account on your phone
B. is linked to the Google account you can access on a computer. (You haven't lost your Google password, it's merely that the device is locked! with a different password *****)
C. You have access through your Google account (e.g. on a computer) to what is called Google Device Manager.
D. Through your access through your Google account, through a computer where you are logged in as your Google account not anyone else's Google account...
E. ... you have access to Google Device Manager for your phone.
F. change the access code that Google Device Manager sets on your phone, to just
and save that change.
G. wait 30 minutes. Repeat, wait 30 minutes before the next step.
into the screen on your phone. It should unlock.
If you have no active devices, your phone really has been taken over... unless you can register your current phone (physically yours) with your Google Device Manager account.
Do any of these methods work:
If your phone is actually registered to another Google account now, then you probably cannot do anything but Hard Reset it and wipe it back to factory issue status. In which case, you have lost all data on the device.