Had this problem before and forgot where I wrote down the solution. The PE circle has disappeared, apparently too large for the scene. I can't remember how to bring it back into the scene. Can someone please help? Thanks!

Are you in Edit mode? The proportional edit circle appears when you activate PE using the keyboard short cut of "o" then grabbing verts to move and circle should appear you can use the scroll wheel to adjust the size of the circle or the page up and page down keys on your keyboard I hope this helps if you had it opened and and it got to big maybe reopening the scene might have the same effect as resetting it. I hope this is of some help

Keep in mind, if you use the wheel to resize it, the circle enlarges as you scroll down.

Also, in the lower left-hand corner of your work area, you will see a pop-up appear which will give you an idea of the size of sphere of influence of your circle: as you scroll up the numbers next to "Proportional size" will decrease and eventually you will see that circle. That frustrated me no end until I figured out a way to just keep the mouse still and operate the scroll wheel until I could see the circle.

Thank you. I think it was the Page Down that fixed it.