once milk supply has been depleted, is it gone for good?
Not necessarily, and psychology has a lot to do with it. If you think it's gone for good, it will have a hard time coming back.
But - and this is documented - there are traditional societies where the mother and grandmother often breastfeed the same baby - the grandmother starts producing milk again, even after many years. That same has happened after the mother suddenly died - female relatives who had not suckled for a long time, were able to feed the baby.
That's unlikely in our stressed modern society. But the point is this: your attitude and your expectations play a very big role here! If you want to start again, try putting the baby to your breast frequently, for short periods, and think ' milky' thoughts.
Visualize that flow of milk letting down and streaming out...