In a space opera soft sci-fi setting, the typical War in Heaven happens as two superpowers go to war and drag everyone into it. Over the course of about 30-ish years, planets were trashed, empires shattered and whole civilizations wiped out.
Refugees ran and hid wherever possible. Most were eventually found and killed. The ones that survived were the ones who landed on un-populated worlds and shut down any technology that could be used to find them.
Then at some point it stops. Anyone alive has no idea when it ended or how it ended. The War in Heaven just seems to have stopped. No winner and the super-powers are just as shattered as anyone else.
Question: How long does it take to rebuild the collective galactic civilization after most of the people are dead and most infrastructure is gone?
Planets were damaged but usually not to the point of permanent uninhabitablity. Roughly on scale with destruction of dinosaurs. Limited evacuations of planets could be achieved, usually early on in the war when there were still military fleets to buy civilian ships time.
In traditional space opera logic, inhabitable planets are common, and most alien species can live happy in a least one Earth like biome environment.
FTL Faster then Light tech can be developed by any civilization. It does not require any unobtainium or element zero. Just a lot of R&D.
People Most survivors are aware that high tech civilization is possible. There is a likely to be people alive in most groups who have middle to advanced levels of technical knowledge. It can be assumed that the survivors typically want to re-build their civilizations because being a hunter gather sucks.
Edit the 1st. The survivors do have tech. They would have the ships they landed in. The ships may have been grounded, submerged( standard landing for some races) or half crashed, but much of the ships remain. They would have shut down every system and completely power off for decades, but some level of functionally can be restored.
Space ports and developed worlds were hit early and hard. Until advanced infrastructure can be re-build, the existing ships are irreplaceable.