Is a woman sinful if she is surprised by a non-mahram man entering her house while she is not wearing a hijab, and what should she do?

It is the Muslim woman's right in her home to remove her hijab. It is not permissible to enter any home except after seeking permission. If a non-mahram man seeks permission to enter upon a woman, she must wear her hijab or sit in a place where he cannot see her. If someone enters upon her without her knowledge, or looks at her, there is no sin upon her, but she must wear the hijab immediately upon becoming aware that he has seen her. For Allah has forbidden women from uncovering their bodies or displaying their adornment to anyone other than their husbands and the mahrams mentioned in His saying, the Almighty: (And not display their adornment except for that which is apparent [of it], and to draw their headcovers over their bosoms, and not display their adornment except to their husbands, or their fathers, or their husbands' fathers, or their sons, or their husbands' sons, or their brothers, or their brothers' sons, or their sisters' sons, or their women).