All Apple keyboards failed and I tried three different ones. This occurred when typing some of the keys on my new iMac keyboard and older keyboards. For example: 7, 8, 9, (, 0, U, u, I, I, O, o, J, j, K, k, L, l fail to respond! The solution is to press the keyboard OPTION key 5 times! Incredible! The Logitech keyboard I purchased at COSTCO obviously did not have this problem. I was fearful I had to send the computer in for service. I found the answer on YouTube where this solution was explained.
Note: When you press the OPTION key 5 times a message comes up on the computer screen “Mouse Keys On”. Press OPTION key again 5 times and screen shows “Mouse Keys Off”.