Kill the languages that loaned them to us so we don't need to pay them back anymore.
There's a name for this idea of a restored English: "Anglish"
Old English covers a period of about 700 years give or take, while Middle English saw radical changes throughout just three or four centuries. Noun and adjective cases, grammatical gender, verb conjugations, they all went away very quickly at the same time that Norman vocabulary was introduced in droves. And I mean droves, it was a huuuuge amount of words. There is a wealth of vocab that people are not familiar with anymore, nor their phonology or mechanisms of derivation. What's a geleafa? What morphemes is adlig made of, and how do you inflect it?
That's the grammar part, but at a social level Greco-Roman vocabulary is entrenched as the language of high civilization while English speakers often see their Anglo-Saxon lexicon as being either down-to-earth or pointlessly archaic depending on whether it has survived. You can see it being referred to as "earthy" here and there.
In practice, I think it'd look like current English but with some different sounds since there is no way you're going to bring back all the old grammar. A lot of proposed words you see out there are simply compounds of two words that are already in use, a replacement rather than truly bringing back stuff. IMO, it wouldn't be much of a revival, really.
Nothing like English. English as we know it today is this weird hybrid between Anglo-Saxon and French, and so much of our vocabulary comes from the latter in one form or another that removing it would knock out half our vocabulary.
Tolkien's English. He deliberately chose Germanic words over French/Latin words, for example graveyard instead of cemetery. The intention was to give the reader a feeling of an archaic world.
Yeah, Tolkien's one of the best examples out there, being himself a linguist and taking a lot from Beowulf (or at least, that's what I've read).