My C and M keys have stopped working.

@skykevver that does sound like a bad keyboard. Replacements are relatively inexpensive. IT appears that nobody ever created a guide on how to replace the keyboard. So you will have to use something like this video (slightly different model) and work your way to the keyboard. Once you are there or get stuck in the meantime, post some good pictures with your QUESTION. That way we can see what you see and hopefully help you further. Of course, you can always use a USB keyboard in the meantime.