He entered ihram for Hajj but was prevented from entering Makkah

Firstly: There is no sin on him for exiting his ihram and going back without completing his Hajj, because he was forced to do that. And Allah knows best about his situation and is Most Merciful to His slaves. He will be rewarded to the extent of whatever he did of the actions of Hajj with sincerity. 

Secondly: If a person stipulates when entering ihram that if anything prevents him (from completing Hajj) he will exit ihram at the point at which he is prevented from continuing, then he does not have to do anything, but if he did not make any such stipulation, then he must offer a hadiy to be slaughtered where he was prevented from continuing, because Allah says (interpretation of the meaning): “But if you are prevented (from completing them), sacrifice a Hady (animal, i.e. a sheep, a cow, or a camel) such as you can afford” [al-Baqarah 2:196]. Then he should shave his head or cut his hair, and thus he will have exited ihram. 

And Allah is the Source of strength. May Allah send blessings and peace upon our Prophet Muhammad and his family and companions. End quote. 

Standing Committee for Academic Research and Issuing Fatwas 

Shaykh ‘Abd al-‘Azeez ibn ‘Abd-Allah ibn Baz, Shaykh ‘Abd al-Razzaaq ‘Afeefi, Shaykh ‘Abd-Allah ibn Ghadyaan, Shaykh ‘Abd-Allah ibn Qa’ood. 

Fataawa al-Lajnah al-Daa’imah li’l-Buhooth al-‘Ilmiyyah wa’l-Ifta (11/350) 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked about a man who did Hajj without a permit, and he was prevented from entering Makkah. What does he have to do? 

He replied: If he said when entering ihram, “If I am prevented (from completing Hajj), then I will exit ihram at the point at which I am prevented”, then he may exit ihram and he does not have to do anything else. But if he did not stipulate that, then he has to slaughter a hadiy because Allah says (interpretation of the meaning): “But if you are prevented (from completing them), sacrifice a Hady (animal, i.e. a sheep, a cow, or a camel) such as you can afford” [al-Baqarah 2:196], and he should exit ihram (i.e., shave his head or cut his hair) at the point where he is prevented from continuing. End quote. 

Majmoo’ Fataawa Ibn ‘Uthaymeen (23/433).