Q. I have been asked by many non Muslims of the reason why Muslim women wear the hijab, in which I am able to explain. However, very often I am asked about the consequences of a Muslim woman not wearing her hijab. Therefore this is my question: 'What are the consequences of a Muslim woman not wearing her hijab?' Can you please give me evidence from Qur'an and Sunnah to support your answer?

A. First of all it must be understood that the law of hijab for women is a direct command from Allah, hence, it has the ruling of being ‘Farz’, or compulsory on an individual basis.
In this regard Allah says in Sura An Nur verse 31:-
And say to the believing women that they should lower their gaze and guard their modesty; that they should not display their beauty and ornaments except what (must ordinarily) appear thereof; that they should draw their veils over their bosoms and not display their beauty except to their husbands, their fathers, their husband's fathers, their sons, their husbands' sons, their brothers or their brothers' sons, or their sisters' sons, or their women, or the slaves whom their right hands possess, or male servants free of physical needs, or small children who have no sense of the shame of sex; and that they should not strike their feet in order to draw attention to their hidden ornaments. And O ye Believers! turn ye all together towards Allah, that ye may attain Bliss.
While explaining the above verse, some scholars have stated that ‘except what must ordinarily appear thereof’ refers to the face and hands upto the wrists. It means that a woman must cover her entire body, but an allowance is given to her to allow her face and hands upto the wrists to be exposed (as mentioned by these scholars). Many other scholars have stated that the face and hands must be covered.
From the above verse, the law has been clearly established that Muslim women must cover her entire self properly, which also includes the covering of the head (which is commonly referred to by Muslims as wearing the hijab).
In Sura Ahzab, Allah says, ‘O Prophet! Tell your wives and your daughters and the women of the believers to draw their outer garments around them’ (33:59).
Here, the Prophet(SAS) has been ordered by Allah to tell his wives, daughters and all the believing women to cover themselves with their outer garments, which also includes the head.
With respect to the Sunnah, it is authentically reported that the Prophet (SAS) said, ‘when a woman reaches puberty, no part of her body should remain uncovered except the face and the hand up to the wrist joint’. (Mishkaat). It is narrated that once Asma, the sister of Aisha (RA) came before the Prophet (SAS) in a thin dress which revealed her body. The Prophet (SAS) turned his eyes away and said,
Oh Asma! When a woman reaches puberty, it is not lawful that any part of her body be seen, except this and this, and then pointed to his face and the palms of his hands’. (Mishkaat).
From the above quotations, it can be clearly seen that ‘wearing the hijab’ is compulsory upon a Muslim woman since it is a direct command from Allah, and that of the Prophet (SAS). Seeing that it is a clear order in Islam, it means that if a woman does not observe the hijab, she would be considered as being openly disobedient to Allah and His Rasool (SAS). This act of disobedience on her part is indeed a grave sin, and brings about the severe punishment from Allah in a manner He sees fit.
With respect to one who disobeys Allah and His Rasool, the Holy Quran says, ‘And whosoever disobeys Allah and His Messenger, and transgresses His limits He will cast him into the Fire, to abide therein, and he shall have a disgraceful torment’. (Sura An Naisa (4) verse 14).
In another verse, the Holy Quran says, ‘It is not for a believer, man or woman, when Allah and His Messenger have decreed a matter that they should have any option in their decision. And whosoever disobeys Allah and His Messenger, he has indeed strayed into plain error’. (Sura Al Ahzab (33) verse 36).
These verses make it clear that one who disobeys Allah and His Messenger is astray and will be punished with a disgraceful torment in the hereafter.
Not fulfilling the law of hijab as commanded by Allah, is also a clear disobedience to Allah and hence, will be included in the punishment meted out to those who are disobedient.

And Allah knows best.