Most likely a contact. I've also faced the same issue not long ago.
In that case, it can be serviced by anyone who has:
a) a bit of patience
and
b) a screwdriver or two.
Note that despite the first picture below, you don't need to detach the key itself.
Also, if someone's Mojo61 is booting but not responding neither to the controls nor to the keyboard, you might re-plug the ribbon cable that goes to the mainboard (third picture).