Very Weird

Yesterday I sent an audio file (voice memo) via airdrop from my iPhone to my iMac (10.13.3). But on removing it from the Downloads folder, in the Dock, the whole finder seemed to freeze, at least I could no longer open any folder, even the HD which is displayed on the desktop. No way of getting into the hard drive to even open utilities. (Nothing in the menu bar was working). I searched everywhere for an answer, but most seemed to involve opening the HD which was not possible. Eventually, I reinstalled the OS and backed up from Time Machine - a long job. Everything was back to normal, so I just thought it was a glitch. Today, I again airdropped the same file - with the same result. I can no longer get into ANY folder. I can open individual apps in the dock (I am typing this in Chrome), but nothing else will open. Any ideas, or should I go through the reinstall process again? I did restart with command-r and ran first aid; no problems found. I hope someone has an answer!