Obviously I disagree, but I can't believe I forgot the #1 easiest way to bootstrap: hydroelectric power. Unless the disaster is so great it destroys all these dams, multiple groups of people have the potential for a perhaps small, but generally reliable source of electricity. From that you can build back everything.

Anyway, there's nothing stopping you from believing it's impossible, and ... what will you and yours do after a sufficiently big disaster? All I know is me and mine won't lie down and die.