Today's prompt is another setup, so here is the idea.
The planet is at war. The technology being used in the war is going to wipe out both sides, life, technology, everything. In other words, if there are any survivors it will basically be a civilization reboot. In order to preserve the race as a whole, one or both sides have created "Time capsules" with hibernating people, and technology. This / these time capsule(s) get(s) lost for a very long time, much longer than was necessary to wait for the effects of whatever weapons used did to the planet to wear off. When they do awaken, things are very different, but they have not changed.