Is something jammed under the key? Try removing the key or blowing under it with compressed air. I have felt this when the kid's laptop keys get crumbs under them (not that I let them eat over it, but I do have to work...) If nothing is there it could be something in the key contact which is just a rubberized button. If it's under warranty get them to repair/replace the keyboard.
Plug in a USB keyboard, then once in change the password to something that can be typed into your broken keyboard? Alternatively you could try use ALT- combinations and see if they work. For example, hold down ALT and press 115 on your numpad to get an 's'. Upper case letters are accessed with ALT-65 (for 'A') to ALT-90 (for 'Z'). Smaller case letters are ALT-97 ('a') to ALT-122 (for 'z'). If you need special characters try starting at ALT-33 (for '!') and work your way up from there or google 'ANSI Codes' for a list of all characters.
Check to see if the caps lock/num lock lights are on or come on when the lock buttons are pressed. If not check to make sure that the USB cord is fully inserted (remove and reinstall). Also if it is plugged into a multi-port hub or another device instead of directly into your computer make sure that those have their USB/power cords plugged in as well.