Why wont a part of my hair grow back where I shaved it? Is there anything I can do to fix this? Any medication out there?

I can't say what is happening without knowing more and seeing the scalp up close. However, one principle usually holds true in situations like this and that is that shaving did not directly cause the changes you are seeing. Shaving only made the apparent differences in hair growth in different areas more noticeable. These differences should again not be noticeable in 2-3 months once your hair grows longer. However, should concerns persist, please see a hair specialist.

There are a variety of reasons this could be happening, you need to see a doctor in person to determine the cause.

It would be unusual for shaving an area once (as opposed to repeated plucking) to prevent regrowth.  It is possible that you had thin areas that were not visible due to thicker surrounding hair.  If this persists after the rest of your hair grows hack, an examination with a hair restoration expert would be a logical next step.