I had the exact same problem with my right hand, i.e. couldn't turn a key;
couldn't turn a doornknob, couldn't lift a coffe cup; couldn't do anything which caused my fingers to have to push downward - the same mechanism as when we shift gears. It turned out it wasn't related to riding, but was due to the fact that I had tinys fractures across the knuckles of my first and middle fingers. I didn't even know I had them, but I had "jammed" my hand in an auto accident. Six weeks later I had my hand x-rayed. Could you have injured your hand and not realized it? My hand healed up fine and there was nothing they could really do about it.