How can I kill Finder if it's not responding?
Answer found here.

Open a terminal, and move the saved settings from Finder into a backup folder just in case:

mv ~/L*/S*/*finder.savedState Desktop


Then, you can Force Quit Finder as usual using Command + Option + Esc, or use this handy terminal command:

killall Finder


Then, assuming everything looks good, you can delete the finder.savedState folder which has appeared on your desktop.