Empress Matilda ruled England for only a few months in 1141, and even then she wasn't officially crowned. She was the granddaughter of William The Conqueror and the daughter of King Henry I, and was married very young to Henry V, Holy Roman Emperor, which is where she got the title "Empress". When she was widowed she married again, to Geoffrey of Anjou, and had three sons, one of whom became the next king, Henry II. Just to confuse matters, sometimes Empress Matilda is known as Empress Maud!
Matilda should have become queen when her father died, but she was abroad and her cousin, Stephen, grabbed the throne. She fought a long and bitter civil war with him and managed to capture Stephen at the Battle of Lincoln, after which she ruled for a few months. But Stephen escaped and she couldn't hold her title, so in the end she made a pact with Stephen that when he died, her son Henry would become king.