Do you mean broken off, or just non-responsive? If broken off, buy a new keyboard - unless removed with care, keys that come off laptop keyboard are often wrecked (the little plastic scissor mechanisms are quite delicate).
If non-responsive, try removing the keyboard and reseating the ribbon cable. The service manual for your laptop is here -
http://h10032.www1.hp.com/ctg/Manual/c00364919.pdf - and contains instructions on how to access and remove the keyboard. If unplugging and replugging the ribbon cable doesn't improve the situation, replace the keyboard.