He entered ihraam for Hajj but was prevented from entering Makkah

Firstly: There is no sin on him for exiting his ihraam and
going back without completing his Hajj, because he was forced to do that.
And Allaah 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 ihraam that if
anything prevents him (from completing Hajj) he will exit ihraam 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
Allaah 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 ihraam. 

And Allaah is the Source of strength. May Allaah 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-Allaah ibn Baaz, Shaykh ‘Abd
al-Razzaaq ‘Afeefi, Shaykh ‘Abd-Allaah ibn Ghadyaan, Shaykh ‘Abd-Allaah ibn
Qa’ood. 

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

Shaykh Ibn ‘Uthaymeen (may Allaah 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 ihraam, “If
I am prevented (from completing Hajj), then I will exit ihraam at the point
at which I am prevented”, then he may exit ihraam and he does not
have to do anything else. But if he did not stipulate that, then he has to
slaughter a hadiy because Allaah 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 ihraam (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).