You asked this same question a month ago in another thread, but apparently never saw the answer I posted there. Here is my answer again. I hope you see it this time, and I hope it helps.
(1) Remove all plugged-in modules and peripherals.
(2) Replace all four batteries with N cells which you actually tested outside the calculator for good voltage, then insert the batteries into the battery holder being careful to alternate their polarity according to the "+" and "-" signs molded into the bottom of the battery holder.
(3) Install the battery holder into the calculator.
(4) Press down the back-arrow (backspace) key and keep holding it down.
(5) Press the ON key for a few moments then release it.
(6) NOW release the back-arrow key.
You should see MEMORY LOST. The keyboard should respond like normal now.
If you don't see MEMORY LOST, remove the battery holder and leave the 41CV unpowered for a few days then try again. If still no joy, and you really did double-check your N cells' polarities, then the 41CV is in need of servicing.
Disclaimer: I don't work for HP. I'm just another happy HP calculator user.