For security reasons Facebook will not let you log into your account without knowing your password. At this point the easiest thing to do would be to reset the Facebook password. This can be done by going to: "www.facebook.com/recover.php" then follow the instructions. This will require you knowing which email you used for your account and the password for that, otherwise you will have to reset the password for that too. Once you have got your new password you can log in again. If you have managed to get your password, but still don't know your email, you can either use the mobile number attached to your account, or the user name (for example 'big-dog-5') attached to your account. If all else fails you will have to create a new account.
Be warned that if you are posting this question with the intention of hacking into someone else's account by, for example, resetting their password. This IS illegal!
Be warned that if you are posting this question with the intention of hacking into someone else's account by, for example, resetting their password. This IS illegal!