How old is your keyboard? Every keyboard/key has a life i.e number of times you type a specific key. It is usually in millions but that million can run fast if you are a gamer or use those keys quite often.
Other reason could be if there is something underneath the key likes bits of biscuits or paper etc. If you think you might have spilled some kind of liquid or there is something underneath the keys you might need to get the keyboard repaired or if you think it is quite old get it changed. (You can also go to youtube and learn how to clean the keyboard yourself).
Read here if you'll need more help:http://www.techyv.com/questions/my-keyboard-keys-are-not-working