Problem: The mouse doesn't work at all.
First, toggle the on/off switch on the bottom of the mouse and see if the light turns green. If it doesn't and your Magic mouse is older, it may be powered by 2 AA batteries, if so you can try replacing them. Otherwise, plug the mouse into your lightning port for 10 minutes and try it again.
Problem: Mouse has power, but the cursor doesn't move.
Try turning off Bluetooth on your Mac, waiting a few minutes, and turning it back on. If that does not work try removing the device from your Bluetooth devices and re-add it. If the mouse will still not connect if possible try another Bluetooth device. If the other device is working on your Mac then you know the problem is likely the mouse.
You can also try resetting Bluetooth on your Mac:
Hold down Shift and Option, click on the Bluetooth icon in your menu bar and select "Reset the Bluetooth Module." If you're running Monterey, you'll need to open Terminal and issue the following command "sudo pkill bluetoothd".
Problem: Mouse scrolling backward.
Check the scroll direction setting for your mouse. Open System Preferences > Mouse, from here check or uncheck the "Scroll direction:" box and try scrolling again.
Problem: Left or Right click not working.
Open System Preferences > Mouse, make sure secondary click is checked off. Then check to make sure it's set to the correct side of the mouse, depending on your preference.