Let $WXYZ$ be a rhombus with $\angle WZY=120^{\circ }$. Let $M$ be the midpoint of $\overline{XY}$, and let $N$ be the point on $\overline{ZM}$ such that $\overline{XN}$ is perpendicular to $\overline{ZM}$. What is the degree measure of $\angle XNZ$?