If you tried enough times and it won't offer you an alternative, as said above, ie if it says enter a Username and password it means your full Gmail address and that password.
Log in to your Google account on a pc and follow prompts to get that Google account password back.
If you are not on Lollipop, Android Device Manager via your Google account may still work. Use the Lock feature from a pc and it will ask you to enter a new pin/pw that will override the old one when the remote Lock command reaches the phone. No guarantee it works any more, but it did for me on KitKat.
www.google.com/android/devicemanager
