I understand that the Crisis of the Third Century broke the empire politically and economically but I don't understand why Latin fell out of use to the point that Latin is considered a dead language. Even when the Goths, Vandals, Franks, and the Moors took over Western Rome didn't its residents have generations upon generations using Latin? Was Latin suppressed?
Here are a few threads on the topic.
Short answer: Vulgar Latin didn't die out. It evolved into French, Italian, Spanish, etc. Classical Latin was kept "alive" by writers and the church for quite some time, and it is still used to some degree in scientific, medical, and legal professions.
It didn't just disappear overnight! It slowly evolved over time into today's romance languages. No language stays the same for a thousand years.
Another thing that should be mentioned here, in addition to Latin spawning the romance languages over time, is the contemporary issue of the Eastern Empire's use of Greek. We can't be certain, but it is very likely that Justinian was the last Roman Emperor to use Latin as a primary language. A Greek-centric empire needed to use the language of its people.
Latin isn't technically considered a dead language. It, like all languages, evolved constantly over time. Latin is more comparable to Old English. People don't speak Old English anymore, but we don't really say Old English died. Rather, it evolved into Middle English, then Modern English.
(specifically, Latin evolved into Vulgar Latin which evolved to the Romance languages we know today).
This is all just a result of children speaking silently differently than their parents did.