So I have been creating this one planet recently. It has naturally occurring water, atmosphere, and alien life already there. It's a nice place that is almost a duplicate of pre-human earth, just with a different cast of lifeforms.

In the story I created, which is set in the mildly far future, the human species as a whole united into one gigantic country with the exception of a few tiny rebels here and there. This super-nation controls several solar systems but is still in need of naturally habitable planets. They noticed this planet and started putting down domed cities (looking somewhat like this).

Now this is the part I need help on. For some reason or another much of the super-nation's population dies. The government falls down and communication between planets is completely cut off. Everything falls into anarchy with small groups all competing for power.

At the end of all of this the planet splits into 20 or so different countries and technology has been reduced to something from the medieval period or before.

Since I am horrible at coming up with ideas, I couldn't think of a circumstance or weapon that could cause such a powerful nation to fall apart in a instant. It needed to not destroy the environment and solely target people in a way that cannot be prevented, even by a highly advanced civilization. It wouldn't eradicate all humans but just heavily reduce and shatter society. Is such a weapon or situation possible?