The keys are electrically positioned on a grid. If one column or one row of
the grid goes out, then all the keys in that column or row will (probably) go
out. So it's not likely to be a problem with the internals.

That leaves the key switch itself (which would require dismantling), or
something keeping the key from being depressed all the way. It could very well
be -- as Scrooge would say -- "an undigested bit of beef, a blot of mustard, a
crumb of cheese, a fragment of underdone potato". Pop the key cap, and see if
anything is caught under it.