Short answers, pulled from the story.
The Prayer Book Rebellion in 1549 was caused by the government's mandate to conduct all church services in English, which erased the cultural identity of the Cornish and Devonian people who spoke no English. This religious shift combined with economic hardship, a poll tax on sheep, and the destruction of monasteries to create a calculated rejection of the new Protestant order imposed by King Edward VI.
The Prayer Book Rebellion began on Whitmonday, the day after the new prayer book was first enforced, in the village of Sampford Courtenay. Parishioners there compelled their priest to revert to the old Latin service, and a confrontation with magistrates turned deadly when William Hellyons was killed.
The rebel forces were led by Sir Humphrey Arundell and John Winslade, while the government forces were commanded by Lord John Russell. Russell brought 160 Italian arquebusiers and a cavalry force of 850 men to suppress the uprising, which eventually numbered some 7,000 men.
At the battle of Clyst Heath on the 6th of August 1549, Lord Russell's troops killed up to 900 bound and gagged rebel prisoners by slitting their throats in ten minutes. Approximately 2,000 soldiers died in the battle, and the rebels were forced to flee north up the valley of the Exe.
The Cornish language rapidly declined during the 16th and 17th centuries because the government refused to translate the Book of Common Prayer into Cornish. By 1700, Cornish had become an endangered language following the association of the language with sedition and the cultural suppression enacted after the rebellion.