I love a story with a happy ending, and this is one.
I was chugging along on my iMac when I wanted to check my schedule to see what was coming up. I've got my desktop set up using Spaces, so all I have to do is type ctrl-→ to swap out the current display for a different one that shows my Calendar and Contacts. But nothing happened. Then I discovered that nothing happened when I tried typing anything else, either.
"Hmmm," I figured, "dead batteries in my wireless keyboard." So I installed new ones. Still nothing. Another pair of new ones. Still nothing. I tried turning the keyboard off and on again. No little green light. Conclusion: dead keyboard.
Those were, in fact, the 1st 2 words out of my mouth after I walked into the Apple Store at West Towne and one of the friendly blue-shirted associates asked what he could help me with. Less than 10 minutes later I was walking out of the store clutching a brand-new, still-in-plastic replacement keyboard, batteries already installed. $69 value. No charge. Yay, Apple!
Now we come to the "some assembly required" part. I set the new keyboard down in front of my iMac, pressed its IO button, waited for its little green light to start blinking, then waved my hands and said "You guys talk to each other now." Alas, they did not. And, of course, without a functional keyboard on which to type my log-in password, there was no way I could fire up my iMac and ask it to please begin the conversation.
Or was there? (Here comes the tip.) It turns out that your computer comes with a "Guest User" account pre-installed for your convenience. I suspect that the average user just ignores it and goes straight into her or his own personal account. But you can log in as "Guest User" without needing a password. I did that, then used my fully functional mouse to select "Bluetooth" from "System Preferences" and discovered that my new keyboard had been beseeching the computer all along for a hook-up. All I had to do was type in the 6 digits displayed on the screen, and presto! the deed was done. Logged out as "Guest User", logged in as "Richard S. Russell", and all was back to normal.
I guess the moral of the story (beyond merely "Yay, Apple!") is that you never know when that "Guest User" account may come in handy, so don't get carried away with your spring cleaning and decide to throw it out.