Hello and welcome to the forums killkill - thanks for joining!!

If you are talking about a screen lock password, then after a fixed number of incorrect entries your phone should prompt you for the password for the gmail account the phone is registered to. If that doesn't work, shut off the phone, log into the gmail account on a website and change the password using Google's password recovery. Once the new password is confirmed, turn on the phone and try using the new password.

The above only works if you have either WIFI or 3G on, if not then a factory reset (which would mean losing any and all data) would be the only way to go.

 

Just realised that you have said that you don't know which account you have it associated with... if you have more than 1 Gmail account, then you will have to try get your password for each account