Mouse frozen in upper corner right

This could be a simple battery charge issue in the mouse or it could have lost its Bluetooth wireless connection. First, plug in the mouse's charging cable to the back of the computer and the mouse and let it charge for about 30 minutes. Then disconnect the cable and see if the mouse works. If it does, fully charge the mouse.

If the mouse still does not work after charging then follow the steps below:

The following will require any USB keyboard and mouse. If you don’t have them, borrow or buy them, they are inexpensive!

1.     Connect the USB keyboard and mouse to the computer.

2.     Open System Preferences – Bluetooth

3.     Un-pair the wireless keyboard and mouse.

4.     Turn off Bluetooth

5.     Restart the computer in Safe Mode per the directions in https://support.apple.com/en-us/HT201262

6.     Restart the computer normally.

7.     Open System Preferences – Bluetooth

8.     Turn on Bluetooth

9.     Re-Pair the wireless keyboard and mouse.

10.  Unplug the USB keyboard and mouse and begin testing the wireless devices again. 

The mouse isn't pairing to the Mac. See the discussion in this topic: iMac Bluetooth Won't Connect To My Mouse and Keyboard.