Henry VIII had specified that his first daughter, Mary, should reign next in the event of King Edward VI dying young.
While Edward was still alive, however, his ministers persuaded him to make a will naming Lady Jane Grey (who had married the son of his Protector, the Duke of Northumberland) his successor to the throne. They wanted to keep England a Protestant country and Lady Jane Grey was a Protestant, unlike Mary who was a Catholic.
Lady Jane Grey was only 16 years old and did not want to be queen when her parents told her on the 10th July 1553. It is said she became sick with fright and fainted.
The people of England supported Mary and so she marched into London and took the throne nine days later on 19th July 1553.
Lady Jane Grey was then imprisoned in the Tower of London until Queen Mary had her executed the following year.