A client’s Mac had a keyboard with an ’s’ key that had stopped working. But we knew it wasn’t the keyboard itself because we tried another one and the same key didn’t work. The client’s password contained the missing key, and it worked on the login screen. Also, ’s’ didn’t work ‘S’ did.

Look in the system preferences. There are a couple of settings that may cause this strange behaviour. The obvious place to look is the keyboard preferences. 
See if the missing key has somehow been assigned to a shortcut.

Keyboard & Mouse pref pane

Another preference pane is the Speech pane, and this is the one where our missing key was found. The client in this case had at some point thought it would be nice to boss the Mac around and had left the pane with the letter ’s’ where the Esc key is indicated in this screen grab.

Speech pref pane

Other panes that can adopt keys for shortcuts include Spotlight, International, and Universal Access.

I hope this helps someone get the use of their keyboard back.