For some reason the Myceneans abandoned their civilization between 1200 and 1100 BC. The populations of their once-mighty cities dwindled rapidly until there was no urbanized culture left on the Greek mainland. Most of the cities were eventually destroyed, and all the great craftsmen of the Mycenean cities faded away when society could no longer support them. How much of their culture they abandoned, we don't know. For the one key element of their culture that they did abandon was writing , and we don't know why. Without writing, they left us no history following the collapse of Mycenean civilization; we have, instead, only five centuries of mystery: the Greek Dark Ages. Also called, the Greek Middle Ages, this period may have been precipitated by migrations and invasions of a people speaking a dialect of Greek, the Dorians. Later Greeks believed this to be the case: in Greek history and legend, the Dorians were a barbaric northern tribe of Greeks who rushed down into Greece and wrested control over the area.
In the absence of archaeological evidence, it seems unlikely that a nomadic, tribal group could so easily overcome a highly efficient, warfare-centered society like the Myceneans. There is, though, no reason to disbelieve the Greeks. The best explanation is that a combination of economic decline and migrations of northern peoples slowly spelled the end of the Myceneans.
From 1200 (or 1150, or 1100, take your pick) to 750, the Greeks lived a fairly sedentary, non-urbanized, agricultural life. Many villages were abandoned, and it seems likely that many Greeks returned to a nomadic life in small tribal groups. Many Greeks in this period took to the sea and migrated to the islands in the Aegean; according to Greek history, they were soon followed by the Dorians.
Not only did the Greeks abandon writing and most crafts, they also abandoned their large commercial network. They virtually stopped trading with Asia Minor, the Middle East, and Egypt; in fact, they seem to have stopped trading with one another as well. Fortunately for the Greeks, none of the great powers had ever been interested in Europe or the Aegean, so the Greek Dark Ages, once the Dorians had settled, were probably a time of peace. This long breathing-space allowed the Greeks the leisure to slowly redevelop an urbanized culture.
Despite the bleakness of the situation, the Greeks began to slowly urbanize in the latter part of the Dark Ages. This early urbanized culture would produce, at the very close of the Greek Dark Ages, the single greatest Greek accomplishment in the Greek view of themselves: the poetry of Homer. Not only are the two epic poems of Homer windows into the distant Mycenean past and into the darkness of the Greek Middle Ages, they are the defining moment in Greek culture; for the Greeks will turn to these poems throughout their history to define themselves culturally, politically, and historically.