I always had a circle when I chose my clone stamp tool delineating the size of the stamp and what area I was choosing to duplicate. The circle would also let me be aware where I was cloning the information.. I changed the size of the stamp tool circle by either selecting the size in the top line or simply using the [ or ] tools.
Suddenly I did not get the circle outline. I was selecting a circle of content to clone but, I pretty well had to guess the size and the content.
Then after about an hour, suddenly the circle reappeared and works just fine.
I know that this must have been user error but, would like to know what happened; so I can get the circle back if I lose it again. The clone stamp is one of my favorite tools in the Photoshop tool bar...
As always, this happened at a time when I needed to get the images of two dogs online in a hurry...