Assuredly and quite by accident, you have turned-off the little green dot that shows snapping in your Inventor sketch environment. But now, how to get it back on? The first thing to check is Constraint Settings from within the sketch environment or the Application Options from the Tools Ribbon tab. But the other day, I had a call that said the settings were correct, but they still were not getting a green dot at the origin point of the sketch plane. Come to find out, the point had not been projected. We added it, and corrected the setting for that as well. See the image below for the whereabouts to check your settings.