I think you would need biological or nuclear weapons. 

Imagine a world war that lasts for a decade or more. That in conjunction with nuclear explosions or epidemics (like airborne Ebola virus), could very well reduce the population enough and devastate economies enough to revert. 

I don't think you would get to the stone age right away, or that it would be a guarantee. There will be people with expertise, but as long as those individuals don't have a means of passing that information to future generations, it will be lost. 

Additionally, if these people are isolated to a region of the world, they may not have access to where the information is stored. 

I do agree with the other commenters that it would be nigh impossible to revert to the actual Stone Age. 

You may find the show the 100 interesting; it handles a similar concept.