Mostly the Arabic civilization(s) next door, which managed to retain (and extend) much (not all) of the knowledge of the Greco-Roman civilizations for several centuries.

[EDIT: Along with various libraries throughout Europe that also acted as repositories for Greco-Roman knowledge ]

> repositories for Greco-Roman knowledge

True, but a lot of that stuff was re-invented only to be later discovered that the Greeks/Romans had done it first.