What was Queen Matilda's last name?

Royalty do not have "last names" like ordinary people, instead they are described as belonging to a "Royal House."

Matilda was a grand daughter of King William I (William The Conqueror) and thereby belonged to the House of Normandy.

Her husband was Count Geoffrey of Anjou, and by him she was the mother of Henry II, the first King of the House of Plantagenet.