Did you clean your keyboard before it doesn't work ?
A problem could be key contacts on the board. When old or dusty key contacts aren't made for those keys who doesn't work. You may open the board and remove dust inside. Don't use any liquid. Just blast dust with air.
Keys are plastic or caouchouc made of and as you should knows those materials aren't condoctors. So there is an additionnal substense on keys (contact side) to make electrical contact when pressed. Old keyboards (not only ATARI ones but also remote controls, calculators, etc) could loose this substense. So the keys doesn't work. You may replace it with paper pen powlder (graphits powlder in fact). Try to paint keys with a paper pen on the contact side and try again your keyboard.
