Ice is a solid substance, so the particles of ice attract one another with strong forces. These forces of attraction hold the particles closely packed in solid ice. The heat which we supply to ice during melting is all used up to overcome the forces of attraction between ice particles so that they become somewhat loose and form liquid water. This heat does not increase the kinetic energy of particles and hence no rise in temperature takes place during the melting of ice. But when all the ice has melted to form water, further heating increases the kinetic energy of water particles due to which the temperature of water starts rising sharply.
