In cartesian coordinates a circle centered at (x0,y0) of radius r is defined by:
(x - x0)^2 + (y - y0)^2 = r^2
So, if you know x, you have two possible values for y:
y = y0 +/- sqrt( r^2 - (x - x0)^2 )
But this is assuming that the point you clicked is close enough to the circle. So you should compare the chosen y with an y on the circle.