My mouse is not connecting to my iMac

If you have a rechargeable Apple Magic Mouse..?

Connect it to the iMac for a minute or two using the supplied charging cable.

see > If you can't connect your Magic Keyboard, Magic Mouse, or Magic Trackpad to your Mac - Apple Support

If you have an older AA battery powered Apple Mouse...?

Dig out, borrow, buy an inexpensive USB mouse to access the Bluetooth setting re-pair the wireless mouse.

with wireless mice and keyboards it's often a good idea to keep some old usb mouse and keyboard in a cupboard for emergencies, need not be made by apple or anything any one would do windows ones too.