Christian missionaries from Rome introduced Latin to the Anglo-Saxons. It went on to be used as the language of the Church and of learning. At first Old English was used for legal documents and records, but after the arrival of the Normans in 1066, Latin was used for these purposes as well.
The Normans brought French, which became the language of the royal court and the nobility, as well as being used in law courts and as an international language of trade by merchants.
It wasn’t until the 1300s that English began to come back into use as a written language for everyday purposes.