Short answers, pulled from the story.
The Kingdom of Romania was officially established on the 10th of May 1881 when King Carol I was crowned. This event followed the Treaty of Berlin which recognized Romanian sovereignty on the 13th of March 1881.
Charles of Hohenzollern-Sigmaringen, who took the name Carol I, founded the modern monarchy after assuming power on the 23rd of February 1866. His rule lasted until the monarchy ended in 1947.
Greater Romania included Wallachia, Moldavia, Northern Dobruja, Transylvania, Bessarabia, and Bukovina by 1920. The Treaty of Trianon assigned Transylvania and Banat to Romania in 1920 while the Treaty of Saint-Germain ratified the union with Bukovina in 1919.
King Michael I abdicated on the last day of 1947 due to Soviet pressure and internal communist movements. His departure led to the proclamation of the People's Republic of Romania and the end of over sixty years of Hohenzollern rule.
The Kingdom of Romania ceased to exist when King Michael I abdicated on the 31st of December 1947. This event marked the end of the state as it existed since 1881 and established a communist regime under Soviet influence.