In contrast to previous reports, where the modification of elastic constants of semiconductors irradiated with heavy ions was related to crystalline to amorphous transition, here we show that hydrogen implantation causes a dramatic modification of the shear modulus of Si at relatively low levels of crystalline damage. To study the system, we developed an alternative and rather general method to determine the shear modulus of the buried implanted layer. We use elasticity theory to link two simple measurements: (i) the wafer curvature to extract the in-plane stresses and (ii) x-ray diffraction to determine strains in the implanted layer.