I suppose you mean "broken" as in not working and not physically broken?
Try scanning for virus malware (they can cause such issues), and installing KeyTweak to remap or refresh the key to character assignments.
just boot into safe mode and see if the key is working there, should also try booting up a linux live usb to ensure it's hardware or software. If it's physically broken you'll have to buy a new keyboard and install it yourself or have the warranty guys or a repair shop do the job for you. In case it's physically broken we'll need the specific model so that we can help you find one. If you don't want to replace the whole keyboard and depending what is broken you'll have to find the key alone which won't be easy or just swap it for one you don't use (an f#, num lock, the right shift/alt/ctrl, the double . or delete button, the home key, just a few examples) However, I doubt you can switch keys if it's actually not working at all.