how to I reinstall Finder

Hi, welcome to the Apple Users Community.

If you hold down the option key and then click and hold the Finder icon with mouse or track pad you should see a list with the command Relaunch as the first option above the icon.

Release the mouse on Relaunch to restart finder or you can simply reboot the machine. I do suggest doing this after your finder is working again.

Also check this out for more troubleshooting of issues - Safe Mode

Best of luck