I accidentally turned Bluetooth off on my iMac. Now my wireless mouse will not respond

If your keyboard still works, press the command + space bar keys to bring up the Spotlight Search. Type in "Bluetooth File Exchange" and press enter when the Bluetooth File Exchange.app result is selected. This will open the app and produce a window telling you Bluetooth is turned off and must be turned on. Hit enter to turn it on.

Then hit enter again on the Finder Window, and a list of BT devices nearby should appear. Use the arrow keys to navigate to your mouse to pair it again.

If the keyboard is also Bluetooth and does not work either, you will need a wired mouse to re-enable bluetooth.

Get a cheap wired mouse/keyboard so that you can turn everything on again. Having a spare, wired set is always a good backup.

I have tried that and Spotlight does not come up. Then my iMac goes to sleep and I have to restart again...I've done this a few times without success

Got it to work.