Identify Extension: The initial length is $L$ and the final length is $(L+l)$. Therefore, the extension (elongation) produced in the wire is $\Delta L = (L+l) - L = l$.

Identify Force: The force stretching the wire is the weight of the suspended block, so $F = Mg$.

Formula for Potential Energy: The elastic potential energy ($U$) stored in a stretched wire is equal to the work done in stretching it. For a wire behaving elastically, this is given by: $U = \frac{1}{2} \times \text{Load} \times \text{Extension}$

Calculation: Substituting the values: $U = \frac{1}{2} \times (Mg) \times (l) = \frac{1}{2}Mgl$