It is a dead ball and a side-out or point is awarded to your opponents....being they never touched the ball.

Since some nets may not extend the full width of the court, any ball passing through the net opening on the first or second hit of a volley will be replayed, unless in the judgment of the referee, the offensive player intentionally caused the ball to pass through the opening. If the action is deemed intentional, a side out is to be called.

A ball passing through the net opening on the third hit or on the serve will result in a side-out.