As weird as it sounds, my 't' key works for login (it is part of my password) but once I am logged in it doesn't work in lower case in any app or at the command line. I have rebooted twice.
I am using a toshiba z835 ultrabook.
The keyboard know the key is being pressed - the backlight comes on and if I keep it pressed I see flickering indicating that's it's being detected, but no 't' appears.
Uppercase T is working ok! Plus when i put caps lock on and do shift-T I actually get a lower case 't' ! Obviously can't work normally with that.