Post History

Depending on the "variety" of hardness, heating the bread slowly can work.  When bread goes stale, it (the proteins, if I remember correctly) crystalizes, and the heat denatures them again.  And you want to do that slowly, since doing it quickly toasts the bread.

I picked it up as wrapping it (foil or a brown paper bag) and heating it at 300°F (150C) for five minutes at a time until it seems usable.

Also, I remember, when consumer microwaves were new, that heating bread there for about a minute turned it absurdly soft, but if you didn't eat it soon it turned rubbery.  Presumably, the technology (and training) has gotten better, but it's beyond my experience.

I have also heard that, for more difficult situations, especially pre-packaged bread, of people sealing a couple of celery stalks in a bag with the bread overnight, though I have never tried that, either.