Mouse stuck on the left hand corner

This usually happens if a) the connection to your mouse is lost or b) the batteries in your mouse are dead, or c) Bluetooth has been disabled on your computer.

Try rebooting and see if that helps. Also, check that Bluetooth has not been disabled accidentally. This might require attaching a corded mouse temporarily so you can navigate menus.

Check this support article from Apple for more information.

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