My Magic Mouse stopped working

WiFi is on but it's not connecting.

Your mouse connects through Bluetooth, not wifi.

Connect a Bluetooth device with your Mac - Apple Support

Location of the charging port on the Magic Mouse 2:

Also note the little on/off switch by the "eye."

This can also be an issue:

Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support

Assuming this is an Apple Magic Mouse, have you connected it to the Mac via the Lightning charge port on the bottom of the mouse, and allowed it to charge?