Has your brush tip ever gone missing so that you are left with just the cross? And that is really a pain when you are using the eraser tool because you have no visual indicator of how big your eraser is. And that's a problem.
I never could figure out why this happened or how to get the brush preview (circle) back and it would drive me crazy. I would even go into settings to make sure that I had opted for Brush Preview or shut down PS and restart, which sometimes restored my brush preview, sometimes not.
What happened was that I had accidentally turned on Caps Lock, and that takes away the brush preview. It's a toggle, so Caps Lock off = circle / brush preview which gets bigger or smaller as you enlarge or decrease brush size. Caps Lock On = cross / no brush size preview.
So if you are missing your brush preview, turn off Caps Lock!