I switch my Apple keyboard and trackpad between two computers all the time -- my personal and work computer. Normally I use a cable to re-pair them, that's quick. But when I don't have a cable, I've been SO stuck, I just can't use the keyboard/trackpad... Today I learned a trick!! Toggling power off and on a bunch! (5 times?) That will force the keyboard to show up for Bluetooth pairing (without a cable) https://discussions.apple.com/thread/255600135?sortBy=rank