If it shuts down and never comes back, yes bitcoin is dead, but we’ve got bigger problems than bitcoin.
If it shuts down but the internet is quickly restored, miners would probably reconnect and start up where they left off. Any smaller forks would drop out in favour of the greater good and bitcoin would be quickly back up and running. Worst case scenario something goes haywire and there would be an agreement to use a new version starting back where the pre shut down left off. Again people would readily accept this for the greater good of restoring the network quickly. It would be pretty fast. No changes required, just roll back the clock a bit to undo anything bad that happened when the power came back on.
I get asked this question quite a bit, that’s how I respond anyways. What do you tell people?