Britain has never adopted Salic Law, which means females were never barred from inheriting the Throne. However, at the time of Henry I's death, England has never had a female Monarch, which was a huge issue since Henry's only remaining legitimate child was a girl, Matilda.
Henry made all his Barons swear allegiance to Matilda and she was set to succeed him. However, soon after Henry's death, his nephew Stephen (aided by most of the Barons) staged an uprising against Matilda and usurped the Throne. Thus, Matilda was never crowned Queen of England. She didn't give up, however, and a civil war gripped the country. Luck switched sides quite often during the war, however after many years of fighting Stephen lost a lot of support and Matilda's chances seemed to be more favourable. Tired of bloodshed and never ending war, Matilda and Stephen made a deal, which stipulated that Stephen would continue reigning till his death but would be succeeded by Matilda's son (future Henry II).