how do you unfreeze a cursor?

Try the key combination Command+Option+Esc to bring up the Force Quit window.

Use the up or down arrow keys to select Finder and then the Enter key to relaunch the Finder. See if that will unfreeze the mouse.

OT

If the cursor is frozen, the first thing you should try is to restart your computer pressing Control, Command and Eject keys, to discard that the computer is frozen. If it doesn't work, hold the Power button of your computer until it turns off, and turn it on.

If this doesn't work, you should consider the option that the mouse isn't been detected by your Mac