$U=\frac{1}{2}\frac{{{Q}^{2}}}{C}\Rightarrow U=\frac{1}{2}\times \frac{{{(60\times {{10}^{-9}})}^{2}}}{2\times {{10}^{-9}}}=\frac{1}{2}\times \frac{3600}{2}\times {{10}^{-9}}=900\times {{10}^{-9}}J=900nJ$