How can I connect Bluetooth on my iMac?

The following will require any USB keyboard and mouse. If you don’t have them, borrow or buy them, they are inexpensive!

1.     Connect the USB keyboard and mouse to the computer.

2.     Open System Preferences – Bluetooth

3.     Un-pair the wireless keyboard and mouse.

4.     Turn off Bluetooth

5.     Restart the computer in Safe Mode per the directions in Use safe mode on your Mac - Apple Support

6.     Restart the computer normally.

7.     Open System Preferences – Bluetooth

8.     Turn on Bluetooth

9.     Re-Pair the wireless keyboard and mouse.

10.  Unplug the USB keyboard and mouse and begin testing the wireless devices again. 

This is Apple's guidance: Turn Bluetooth on or off on Mac - Apple Support

What exactly did you do to "disconnected Bluetooth"?

You'll need to dig out, buy or borrow a wired keyboard and mouse to access your Mac and reactivate the Bluetooth functionality.

If you have one of the Apple Magic Keyboards that is rechargeable, you can use it temporarily as a wired keyboard by plugging it into your Mac using its charging cable (and a USB-C to USB adapter if necessary).

You'll still need a non-BlueTooth mouse. The charging port on the rechargeable version of the Magic Mouse is on the underside of the mouse, so the mouse is useless when the cable is plugged in.