According to Torah law, married women must cover their hair whenever they are outside their home. A woman who fails to do so forfeits her kesubah and should technically be divorced by her husband (Kesuvos 72a; E.H. 115:1-4). Mishnah Berurah 75:14 and Beiur Halachah say it must even be covered inside the home.
Since the hair must be covered, when it is not covered it is considered an ervah, an uncovered area. No male may recite Kerias Shema, pray, recite a blessing, or learn Torah when the uncovered hair is visible to him (O.C. 75:2. This halachah applies to one’s own wife, sister, mother, etc. as well.). Accordingly, if such a person happens to be at the Shabbos table, Kiddush may not be recited.
The Mishna Brura states: even if it is the custom of this woman and her friends in that place to walk with uncovered hair publicly, in the way of immodest people, one may not recite prayers in view of her hair.
Aruch Hashulchan 72:7: “Now let us come and cry out regarding the immodesty of our generation, because of our many sins. For many years (some) Jewish women have been neglectful regarding this transgression, and they go with uncovered hair. All which they (the leaders, rabbis) have screamed about this has not helped or accomplished anything. Now the plague has spread, that married women go about with their hair just like unmarried women! Woe to us that such has occurred in our days.”
Anyone who says a married women can leave her hair uncovered in a rasha.