Probably filled up with dust.Try tipping it upside down and lightly tapping it.I have seen Windows compat keyboards new for $5
My desk is NEVER without a can of Compresses Air. Great for blowning crap out of my keyboard. (I'm a smoker, and ashes ALWAYS fall into mine!) But, Trotsky is right. Check it with ANOTHER keyboard if possible. If that doesn't solve the problem, it COULD be a Chip gone south.