I'm aware that, at least from all outstanding evidence, literacy via Linear B in Mycenean Greece was primarily confined to high level administrative and accounting purposes, but it is still a striking difference in my opinion from the culture that followed, where there is no evidence of writing. Have there been other cultures over the years where, following some sort of collapse, literacy was completely lost?
I'm not referring to, say, a people being conquered and having the indigenous form of writing and the indigenous administration replaced, but rather a situation where following the disappearance of writing there was nothing to replace it for some extended period of time.
Cypro-Minoan, a script that represents the native Cypriot language, slowly stopped being used following the Bronze Age collapse, but was replaced with the new Cypriot syllabary that represents the Greek language.
The Zapotec script appears to have disappeared following the collapse of Monte Alban although other writing systems such as the Mixtec semasiographic system continued in use. More interestingly, James C. Scott in The Art of Not Being Governed argued that many highland groups of southeast Asia gave up writing and recorded the loss of writing in myths-for example, myths claiming that they once possessed books before said books were accidentally eaten or stolen. This argument is in some sense an inversion of the classic claim that "with civilization comes writing"; according to Scott's argument many of the highland peoples of Southeast Asia are escapees from the state and in this line of thought giving up writing is less a symptom of abstract decline than a pointed rejection of the tools of state control.