it depends on how hard and with what and where you're hit. By hit I'm thinking you mean getting punched. . It actually stings more to get slapped in the face then punched in the face. A punch is more of a dull ache than pain. But getting hit is always a bad idea.
Crunches will help you to get abs however, you will not be able to see them without losing weight. If you have a layer of fat over your abs, no amount of crunches will give you the abs that you are looking for.