How do I reset the password on the loading screen?

It's not the screen where it has a person's name and then you type in the password to go to that person's account on the computer. It's right after the black screen with the computer logo and then goes to say 'Type the password and... ---press blah blah blah'. Also, it won't let me type in the password (because I know what it is). I've checked that my keyboard is securely plugged into the modem (whatever it's call) in the USB in the back. It won't let me go anywhere until I can type the password in, which is the cause of my dilemma.