By mistake I turned off my Bluetooth on my IMac on can I turn it back on?

Get a wired mouse and keyboard and use them to turn Bluetooth back on. (Wireless ones that use proprietary USB plug receivers will also do - they appear to a computer to be USB devices.)

If the wireless keyboard in question is a Magic Keyboard that you recharge by plugging it into a Mac, you can plug it into the Mac and use it temporarily as a wired keyboard until your Bluetooth settings are fixed. I don't think that the Magic Mouse can be used as a wired mouse. The charging port is on the underside of the mouse; when you plug in a cable, the mouse becomes unusable.

Plug in a usb keyboard and mouse ( they are cheap!!!!!) and use em to turn BT back on! Easy peasy!

If your Mac is unlocked, and is not asking for a password, you probably just need a mouse to turn Bluetooth on.

It may be possible to turn on Bluetooth with just a keyboard (even a wired connection to an Apple Magic Keyboard). See: How to turn Bluetooth or Trackpad without… - Apple Community