I get the bluetooth keyboard to work again by following the computer's instructions.
Upon restarting, if the keyboard isn't connecting, iMac tells you to turn off the computer via the power button on the back and then push it again to start up. The screen lights up black with a menu about password reset and offers three options. The third option refers to a disconnected keyboard and turning off the encrypted FileVault to get the keyboard to reconnect.
The bluetooth keyboard ONLY disconnects upon a restart or power up (after being shut down). This has only been happening the last 6-8 weeks. I want my FileVault encrypted, eh? Why would this cause the keyboard to disconnect all of a sudden?