It seems like your post is actually asking two different questions: (1) how can a language disappear and (2) how can people forget how to read a particular type of writing?

Languages can disappear for many reasons, but one of the most common reason for the disappearance of a language is actually just that it has changed over time. Languages are not static and sometimes, changes in a language can make them almost unrecognizable. For example, the Great Vowel Shift, which started in the 12th century and didn't fully resolve itself until the 18th century, changed the way English speakers pronounced their vowels, so that Chaucer's English would sound strange and sometimes incomprehensible to a modern English speaker.

Sometimes languages die out when their native-speaking population intermingles with another group of people and that second group's language becomes more dominant. That's what happened (or nearly happened, depending on which sources you find credible) with Cornish - native, fluent speakers of Cornish had basically died out by the early 19th century and most children in Cornwall were educated in English until the revival of Cornish in the early part of the 20th century.

Sometimes you have catastrophes that wipe out a population, which can affect both the spoken and written forms of a language. Historians think this may be what happened in the case of Linear B, which was a script used to write Mycenaean Greek. The last writings in Linear B are pretty well pegged to around the time of the destruction of the Late Minoan II palace in about 1400 B.C.E., so the hypothesis is that the two events are related.