If you are usin MAC, THEN WELCOME TO THE CURSE OF STEVE JOBS.
This a common problem on Mac, most often while using Photoshop, but involving, to some degree, all apps.
Several techniques work - all temporarily:
1) RAPDILY DRAG UNSEEN CURSOR TO LOCATION OF DOCK AND THEN BACK TO IMAG. NO NEED TO CLICK ANYWHERE
2) PRESS CMD+TAB and cycle thru apps back to LR.
There are others that I don't recall.
There are over 50 pages of postings on the apple fora regarding this problem.
If you are not on Mac I can offer no help.