Generally fairly easily, though it's a bit hard to tell without actually
looking at it. Usually it means the contact pad has worn away or has *** on
it. Cleaning the contact pad is easy enough, you just need to disasssemble
the keyboard and clean using some *** and a swab. If the pad is worn
away or trace cracked, using a conducting marker pen like this or similar
Not sure how long that fix would last though.
Alternatively, swap the whole keyboard for a working one ;-)