In order to understand the possible cause of a 'right side stomach pain', it may be helpful to know the organs and structures that exist in this region of the body. Diseases, infections, or irritation of any of these organs will lead to a right side abdominal pain.
At that one instant, its velocity is zero, even though its acceleration is still 9.8 m/secÂ². But velocity cannot REMAIN at zero for any length of time if the acceleration is nonzero.
It would only have zero acceleration if it stayed at rest at the top for some finite period of time, so that you could say that its velocity remained the same -- zero -- for, say, a thousandth of a second, and during that period the rate of change of velocity, the acceleration, would then of course be zero.