if a key is not working, sometimes cleaning it may help. we invite you to remove the key in question and clean the bottom of it. you can also gently press the rubber part underneath the plastic key, to see if the issue persists. note: removing a key will not void your warranty. this will happen only if you remove the entire keyboard. here's a guide to follow from ifixit.com:
if the key is indeed broken you may need to replace the keyboard module. here is a guide for that:
please be aware we do not sell individual keys, but you can purchase a new entire key set from us. if you wish to do so, please submit a request.