This can be because of a keyboard error. Use the onscreen keyboard which can be activated from the ease of access icon on the login screen. Click on Ease of Access > On Screen Keyboard and enter the correct password using the onscreen keyboard I you are using a USB keyboard, just disconnect it clean the contact area with tissue paper and reconnect the USB cable. Also do not push the USB end completely into the slot. USB connectors work the best when they just about fully inserted.