How does a computer user log into their computer?

well there are many different ways a person can log into their computer depending on operating system. in a windows environment you would have to enter the user name and password or just password if it is password protected. on mac osx just the password and in a Linux environment it would be similar to windows. im guessing you cannot log into your machine because it is password protected and you either forgot your password or someone made one for you without telling. in either of those cases...... i would really need more information than your basic question