It happened to me too on one of my five High Sierra machines after the second security patch for the root account security hole. However, I don’t know if that was just a coincidence or not.
It fixed itself after a reboot. Have you rebooted?
I had something similar the other day. Trying to access the Applications folder from the Dock and nothing would open. I tried to open the Downloads folder from the Dock and again it would not open. I browsed to the Applications folder using Finder and it showed as being empty. I fired up Launchpad and all of the applications were there.