I don’t know what I did, but even when I have proportional editing enabled it doesn’t show the circle for the proportional editing influence anymore. How can I get it to show it?
Depending on the size of your scene it may be too small or too large to see. When you do a transform you will see the size of the proportional size on the 3d view header. Make it smaller or larger as appropriate.
In addition to what Richard said it’s possible that your pivot point, which is what the circle will be drawn around, is far away from your viewing area.