how to connect wireless keyboard to iMac so can type in fields

using iMac w/safari, OS X el capitan 11.10.4, suddenly can't get any typing to show in any field. fresh trackpad batteries, fresh wireless keyboard batteries. have just spent an hour on hold for apple, then another 1/2 hour w/a tek guy, then phone cut off (?). tried to connect iMac to keyboard, or keyboard to iMac- but it won't "take"- just keeps deleting whatever have clicked in system preferences. help, please! thank u!