Complex sentence: On 25 May 1555, Henry II of Navarre died, and Jeanne and her husband became joint rulers of Navarre until 1562, but on becoming a widow of the Navarre King Consort Antoine de Bourbon, she kept on ruling as a sole Queen regnant until 1572.
Split into simpler sentences: On 25 May 1555, Henry II of Navarre died, at which time Jeanne and her husband became joint rulers of Navarre until 1562. On becoming a widow of the Navarre King Consort Antoine de Bourbon, however, she continued to rule as a sole queen regnant until 1572.