I have A1255 keyboard (wireless) and 80% of the keys (mostly letters) stopped working. I opened the gray back compartment and it turned out that the main ribbon holding thingy was in the wrong position (up). I slightly tucked the ribbon and put the holder in the down position.
I had previously disassembled the keyboard and must have missed some step putting it back. Or my cat dropped the keyboard from the table and dislodged the ribbon. Or likely both. Now it works.
Thanks @craiglapp2000 for pointing into the right direction with your comment.