This happened to a professional. It could happen much easier to you if you are untrained.
Contrary to what leo said, there are three-four types of roundhouse kicks, depending on if you consider the last a roundhouse.
One is what I call the Karate roundhouse, as leo said, it hits with the ball of the foot. Its best to kick the ribs, knee, temple and jaw with this kick.
The next is what I call the Taekwondo roundhouse ( I learned it in taekwondo), although almost every standup style uses this one. It hits with the instep of the foot, there's a knuckle there that fueses when your around 18 years of age.
Finally, the last mainstream roundhouse is the most common, what I call the Muay Thai roundhouse. It hits with the shin. Very strong. Very powerful IF YOU DO IT RIGHT. Only a good muay thai teacher can show you how to do this. If I tried to explain it on here, I could be responsible for you breaking your leg like Silva.
There is a final rare kick from shou shu kung fu. Its a cross between a side kick with a roundhouse. It is basically a low roundhouse that hits with the blade of the foot to the knee.
Here are the three roundhouse kicks (you won't find the Shou shu one online):
There are two types of round house kick. One hits with the ball of the foot and one with the instep. They can be done with either leg and from any stance. There is no way to teach this, outside of class.
Just to back Leo L up, you can not be taught this or any other technique from a keyboard.
Go ask your instructor.
If you have no instructor, go find one before you teach yourself too many bad habits.
