Short answers, pulled from the story.
The Prayer Book Rebellion was triggered by Lord Protector Somerset's introduction of Protestant religious reforms in 1549, which threatened traditional Catholic practices and the Cornish language. Economic pressures from a poll tax on sheep compounded these tensions, leading to widespread resistance among local populations.
The rebellion began after Whitsunday 1549 when parishioners in Sampford Courtenay forced their priest to revert to old services. The conflict concluded with the Battle of Clyst Heath on the 6th of August 1549, where rebel forces were decisively defeated.
Cornish rebels gathered at Bodmin under the leadership of mayor Henry Bray and landowners Sir Humphrey Arundell Helland and John Winslade Tregarrick. Devonian knights Sir Gawen Carew and Sir Peter Carew negotiated with the rebels before joining the fight against government forces.
An estimated 5,500 people died during the Prayer Book Rebellion, including casualties from battles such as the Battle of Clyst Heath. Lord Russell's troops killed up to 900 bound rebel prisoners by slitting their throats within 10 minutes according to chronicler John Hayward.
Following the rebellion, the English Government associated the Cornish language with sedition and backwardness, leading to the suppression of any translation efforts. This failure to translate the Book Common Prayer contributed to the rapid decline of the Cornish language throughout the 16th and 17th centuries.