How to lose knowledge? It's easy. Most knowledge is known only to specialists in the field. I don't know how to choose a catalyst for a particular reaction, or a turbine blade. Most other people won't know how to design, say, an intelligent sensor with WiFi network capability. There were probably a number of people in Iraq in 2000 who could do these things, and other similar tasks. But pronbably not now- there's no need for them, and no use for them, so they've gone elsewhere.

When society breaks down on a global scale, there's no use for the specialists, their knowledge isn't passed on, and if not preserved (in libraries say)is lost. Any great change- a change of religion say- can have similar effects. There was simply no need for handling masonry on that scale once bronze age "Britons" or Egyptians had lost their megalithic obsessions.