In 1951 the health of King George VI was declining quickly. His attendants had already drafted up the accession declaration just in case. It was a good thing they did because they would be using it soon. George VI was discovered dead in his bed on February 6, 1952 at Sandringham House in Norfolk. Coronary thrombosis, a type of heart disease was the cause of death.
Elizabeth immediately flew back from Kenyan where she was on her way back from a tour of Australia and New Zealand with Phillip. Elizabeth II took control of the United Kingdom, Canada, Australia, New Zealand, South Africa, Pakistan and Ceylon, when she took the crown. Those have since changed as some have become independent or Republics of their own.
She is the second longest ruling British monarch. The first being her great-great grandmother Queen Victoria at over 63 years. Queen Elizabeth has been ruling for 52 years this year. I highly recommend Elizabeth the Queen: The Life of a Modern Monarch by Sally Bedell Smith for those interested in learning more about the queen. I reviewed the book here in 2012.