The year 536, is the beginning of the Dark Ages and then the Justinian Plague. What caused the Dark Ages? My guess is volcanic ash darkening the skies for a few years that caused crops to fail, etc due to summers without sunlight. It literally was the Dark Ages.
Same thing happened in New England in 1816 - the year without a summer. Freezing temps every month of the year.
The cold lasted 10 -15 years, but the sun was dimmed for only 18 months, according to Byzantine accounts:
"For the sun gave forth its light without brightness, like the moon, during the whole year" - Procopius
I have also seen speculation that it was a very large bolide strike. That would have blown ash far higher into the upper atmosphere to make it last for a decade. What do the Greenland ice cores show?? Tambora did have a major impact on US NE climate in 1816 and elsewhere in the world too. I had also wondered why the empire of Charlemagne did not continue after his death. Then I read in a book about nutrition that in the hundred years after his death in 814 there were 33 famines some lasting 2 and 3 years. Two causes were ergot fungus on rye in northern Europe, and another fungus on wheat to the south because Arabs had introduced the barberry plant which is the alternate host to something that kills wheat. Below is an article on all kinds of these plant killers.
http://www.botany.hawaii.edu/faculty/wong/BOT135/Lect08.htm