You can reset the circle shape to full (instead of arc or segment) by clicking on the last button in the circle tool's tool bar at the top (when the ellipse/circle tool is selected, and the object that is selected is a circle).
It could also be that you selected a pattern fill with vertical stripes. Then all you need to do is to set the fill to 'flat color' or 'no paint', depending upon what you want.
Something doesn't work? - Keeping an eye on the status bar can save you a lot of time!