Fix your sketch with Repair Sketch
you can also find it under Tools > Sketch Tools > Repair Sketch.
How to Repair a sketch?
Example: let us consider the given Sketch
when we create a new Extruded Boss feature with this sketch, we notice that no preview is shown:
Also, in the Boss-Extrude FeatureManager, the Selected Contours box is automatically selected. This is a good indication that there is a problem with the sketch.
A Repair Sketch Dialog Box appears and we can see that the tool found two problems in the sketch. A magnifying glass is pointing out the first of two problems. Also, the dialog box is telling us that the first problem is a Two Points Gap, as can be seen in the magnifying glass. The gap is noticed by the tool, because the gap is smaller than the number entered under Showing gaps smaller than. This issue can easily be solved by dragging one endpoint to the other one, so the points merge. This can be done inside the magnifying glass. After this repair, we press Refresh in the Repair Sketch Dialog Box. This re-runs Repair Sketch and the screen will look like this:
In the dialog box, a description of the problem is given: Three or more contour segments meet at this point. Normally, in a closed contour maximally two contour segments can meet at one point. In the magnifying glass, we can see that 3 lines meet in one point. This time it can be easily fixed by deleting the unwanted line. Finally, we press Refresh again and the Repair Sketch Dialog Box returns with the message that no problems were found:
Now we can close the Repair Sketch Dialog Box and create the Extruded Boss feature successfully!