Move the mouse and a gray box will appear in the middle of the screen, prompting you for a login name and password. If all goes well, the screen should go blank, and soome windows will begin to appear on the screen.
Passwords are case sensitive.
You may have moved the mouse out of the window you were typing in while you were typing. Unlike with Microsoft Windows or on a Mac, you can only type in a window when the mouse pointer is inside of the window. The highlighted window is the currently active one. Make sure your mouse is on the window to type in it. Type yppasswd and press return.