Lightroom unresponsive and disconnected from photo catalogue after using Lightroom duplicate application

I have Lightroom classic 15.4.1 .  I used the find duplicates application to create stacks of  duplicate photos.  The stacks of duplicate photos were displayed.   I then selected the option to remove duplicate photos from the hard drive.   Now it would appear that the "duplicate application" has become unresponsive.  "No duplicates found " is shown on the main Lightroom screen. In addition the photo catalogue is no longer connected to Lightroom.  The thumbnail of the Picture file has a small black/grey exclamation mark by it.    My photos are still all on the computer’s hard drive.     HELP !   How to I return Lightroom to its usual state and reconnect it to the photo catalogue?