The majority of people don't use unique passwords for everything they sign up for. Try out any passwords you can remember using for anything and see if they work.
You sign into your Gamertag using the linked Windows Live account. If you can't remember the password, you could always try resetting it. To reset the password for your Windows Live account (and by extension, your Gamertag password) you should go through the password reset process on a Windows Live website (such as Hotmail), found here.
Hope this was helpful!