  We explore the mechanism responsible for the ergodicity breaking in systems
with long-range forces. In thermodynamic limit such systems do not evolve to
the Boltzmann-Gibbs equilibrium, but become trapped in an out-of-equilibrium
quasi-stationary-state. Nevertheless, we show that if the initial distribution
satisfies a specific constraint - a generalized virial condition - the quasi-
stationary-state is very close to ergodic and can be described by Lynden-Bell
statistics. On the other hand if the generalized virial condition is violated,
parametric resonances are excited, leading to chaos and ergodicity breaking.
