The solid to liquid phase transition is the melting process of ice.
Any phase transition occurs due to a change in kinetic energy of the participating particles.
If the system is in the solid phase and the kinetic energy is sufficiently increased, the system changes from solid to liquid. In the solid phase, the network strives for the lowest energy conformation. The geometry leads to a rather open hexagonal structure, each of the four bonds representing a lowered overall energy. When the average kinetic energy is raised, the additional jostling begins to destroy the open hexagonal structure. Paradoxically, this allows the molecules to move closer to each other, making and breaking hydrogen bonds much more rapidly.