Apple wireless mouse isn't working

Please see: Writing an effective Apple Support Communities question - Apple Community

Have you turned the mouse off and then back on to see if it will reconnect?

Have you restarted your computer lately?

Have you plugged the mouse into the computer lately to recharge it?

Has this mouse ever worked with this computer?

Set up your Magic Keyboard, Magic Mouse, or Magic Trackpad with your Mac - Apple Support

Recharge the built-in battery in your Apple keyboard, mouse, or trackpad - Apple Support