[chrisco255]

Probably a number of things: the Roman empire collapsed so infrastructure fell to ruin and it became much more difficult to travel and trade across long distances. In addition, with the collapse came a lack of protection from an organized military, Rome itself was sacked multiple times but communities would regularly get invaded and raided by bandits and various tribes and gangs. That's perhaps why we see the development of castles in the middle ages, as a defense mechanism for an age in which each community was more or less on their own.

There was also a major volcanic winter in the 500s that triggered global cooling and caused droughts and starvation.

There was also the Justinian plague in the 500s which was supposedly worse than the Bubonic plague.

https://en.m.wikipedia.org/wiki/Volcanic_winter_of_536

https://en.m.wikipedia.org/wiki/Plague_of_Justinian

Plagues and droughts have killed off more than a few civilizations, but seeing that these occurred in the wake of the collapse of the Roman empire, it probably accelerated the gradual decline that would have occurred and maybe prevented any sort of recovery.