  Externally driven glassy systems may undergo non-equilibrium phase
transitions (NEPTs). In particular, ac-driven systems display special features,
like those observed in the vortex matter of $\mathrm{NbSe_2}$, where
oscillatory drives reorganize the system into partially ordered vortex
lattices. We provide experimental evidence for this dynamic reorganization and
we show an unambiguous signature of its connection with an NEPT driven by ac
forces. We perform a scaling analysis and we estimate critical exponents for
this transition. Our results share similarities with some
glass-to-viscous-liquid NEPTs and invite to search for similar physics in other
elastic disordered media.
