Imac mouse not turning on

Hello, Deb320.

Make sure your mouse's battery has been charged. Plug it into the USB port of your iMac or plug it into a USB wall charger for an hour to get that started.

Make sure the power switch is turned to "On". There will be a green indicator at the switch when it is.

Then please have a look at this support article –

Set up your... Magic Mouse... with your Mac - Apple Support:

https://support.apple.com/en-us/HT201178