Old English became slowly formed into MIDDLE English after the Norman Invasion. 1066 is often marked as the END of the Old English era, though it was a slow change into Middle English. In religion the language was most likely Latin, and in Politics and a good bit of literature the language in use was literally the lingua franca: Old Norman French.
Later of course it formed slowly into the Middle English of Chaucer. You have some interesting texts such as Brut which show the transition, since it had heavy Old English vocab, yet it was written after the invasion