In a world where fuel are resources are what dictates the wealth of the nation; peace is once again broken. With globalisation the alliances are bigger, warcraft is on a different level like never before — all people of every country in the world are involved.
With different weapons of mass destruction in a moment of despair the losing side unleashes its arsenal of weapons forbidden by international laws: nuclear, bio, anything goes. The rich countries die from the weapons, poor countries die from lack of support and complete pollution of drinkable water.
Multiple new volcanos would form at ground-zero of some nukes thus destroying the architecture of largest cities.
All survivors on Earth are dead, those who survive the terror of war, escaped the nuclear bombardment, haven't died from trauma or injury would succumb to the bio-weapons such as virus (it's possible to create virus to live for X generation and have a self-destruct gene) and/or radiation.
The wealthiest of people would take to space in space-pods and live there for Y years until it's safe to return to Earth once again. Their children would have very limited knowledge about the world as the parents cannot teach everything ( and no more Internet on Earth ). The children's children would know even less.
As time goes by the technology would fade away: book rot, CD-ROM decay, hard-drives and all.
The space-pods would come back to Earth by auto-pilot but the children would be ill-prepared for life. Some would survive, most would die and thus the Earth's population is down to some 1000–5000 people with no skills and no language that would describe the world they'd have to live in.
The old technology would probably be buried by a plaster of soil and some items that could be found would be useless. Would you be able to play back a Dandy cassette if you have nothing but stones and a couple of trees?
The advanced technology of the space pod would wear out from decades (millennia) of use by the survivors and thus only durable items like hammers and crowbars would remain.
And there you have it — a primitive society.
If you want to return the people to pre Homo Sapiens state, make the bio-viruses dumb-down people but make the people to either ultimately develop an immunity or have the virus self-destruct after M number of replications.