The show, Vikings, take place around the year 800 CE. There are Roman ruins in the show and the King of Wessex mocks how everyone thinks the ruins were built by "giants" and how the other must not know the truth about the Roman Empire.
This seems imply that no one (at least in England) knows what the Roman Empire was, even the Nobles don't seem to know.
Did people really forget about the Roman Empire and if so when did we "rediscover" it?
You might be interested in my answer here, which argued that educated individuals were very aware and in awe of the Roman empire, especially because at that point the empire still existed! I only covered events up to 700, but I don't see any reason why this would change over the next century. I mean, that's when more and more Anglo-Saxons began to travel to the Continent to preach or to travel to holy places, at the same time as when the Carolingians were consolidating their empire and promoting learning (one of Charlemagne's advisors was Alcuin, someone he headhunted from York!). I don't think it would be stretching the evidence to say that all the movers and shakers at this point would have been perfectly aware of the Romans' legacy, both in Britain and in Europe as a whole.
Wasn't the situation more that he had all that art stashed away, but the Church was the only group with people literate in Latin and found the pagan Roman objects pretty detestable?