Kurzweil Mark 110 - need help with key that no longer presses down

Hi - I have a Kurzweil Mark 110 Digital Piano (circa 2001). Very recently the B key (above middle C) got stuck in the up position - I can no longer push down on this key. I cannot find a service manual (without going through rather shady looking PDF redistribution sites) for this model online. I am fairly handy but it is not obvious to me how one would go about inspecting underneath the keys to see what the key no longer depresses. All the other adjacent keys function normally.
 I believe the keys are made by Fatar, based on what I've read online.  If anyone has experienced a similar problem and found a pathway to resolve, please share :) - thank you!