Sorry, did not see that one, too many atheists here..
Ie a natural disaster struck them, like Noah´s flood(Atlantis civilization was wiped out), it came during Yu the great´s time -
Yu the Great - Wikipedia, the free encyclopedia.
There could be huge earthquakes, a terrible disease hitting on evil people(they have massive karma so it is very easy to pinpoint such people from other dimensions) or comets, anything to wipe out the civilization when it became too bad.
Was it not said by Swedenborg that the old Egyptians were extremely advanced technologically(see the saucers on their engravements) but also very wicked so they were wiped out.
Wish everyone good luck in their quest for higher truth.