It gets transformed or conserved.
First of all, energy can never be "lost"; it is either transformed or conserved.
When a solid has no forces acting on it, the particles are stored, as potential energy. Now particles are always moving (even in 0
However, if the solid is let's say, heated up, then the particles become transformed from potential to kinetic energy + potential energy. Eventually, the forces of attraction cannot hold the particles in place and the solid melts.