Bismillaah
Allaah 'azza wa jal says in Suratul Anbiya (21):

لَوْ يَعْلَمُ الَّذِينَ كَفَرُوا حِينَ لَا يَكُفُّونَ عَن وُجُوهِهِمُ النَّارَ وَلَا عَن ظُهُورِهِمْ وَلَا هُمْ يُنصَرُونَ
If only those who disbelieved knew when they will not be able to ward off the Fire from their faces, nor from their backs; and they will not be helped. [21:39]
The word for ward off is ‘yakuffoona”, “يَكُفُّونَ“, and this is from the root word “ka-fa-fa” or ‘kaff’, which means palm/hand. So the disbelievers will not be able to put their hands out, avert or prevent themselves from the punishment. Imagine, if something is coming towards your face… what is the first thing you would do? Cover your face with your hands, right? The disbelievers will not be able to do this. They will not be able to avert or obstruct the fire from their faces, may Allaah ta’ala protect us from it, ameen. And Allaah ta’ala continues, “nor from their backs”, وَلَا عَن ظُهُورِهِمْ “, Now is something came to your face and you were not able to cover it from your hands, what would you do then? Turn around and protect yourself with your back, right? The disbelievers will not even be able to do this… “and they will not be helped”… there won’t be anyone, no helper or assistant to come and save them from the punishment.
Allaah ta’ala begins this ayah with ‘law’, if only, and these types of ayaat are throughout the Qur’an. Read the ayah again, Allaah azza wa jall doesn’t answer what would happen if the disbelievers knew this, because the answer is left up to us and it is understood. The ‘if only’ in this ayah carries out two meanings:
Firstly, if only the disbelievers knew… they would not ask for the punishment. They wouldn’t be hasty. They would not say the statements they say.
Secondly, ‘law’ can be understood as ‘harf at-tamannu”, a statement of hope of wish. Like saying “Alas”, “I wish”, so ‘if only they knew’.
Allaah azza wa jal continues:

بَلْ تَأْتِيهِم بَغْتَةً فَتَبْهَتُهُمْ فَلَا يَسْتَطِيعُونَ رَدَّهَا وَلَا هُمْ يُنظَرُونَ

Nay, it will come upon them all of a sudden and will perplex them, and they will have no power to avert it, nor will they get respite. (40)

-”it” in this ayah can mean the fire or the Day of Judgment…It will come to them “baghtah”, unexpectedly, suddenly, without any alert. Then Allaah subhanahu wa ta’ala says, “ فَتَبْهَتُهُمْ “, “then it perplexed them”. This word is from “ba-ha-ta”, and it literally means to bewilder and overwhelm someone. It also means to astonish and shock someone in such a way that they cannot see anything. It will render the disbelievers speechless, because they are not expecting it at all. They will not be able to repel the punishment or the Day of the Judgment and they will not be given respite. What does respite means? It means that they will not be given the chance to repent, to reform or offer an excuse to Allaah subhanahu wa ta’ala. The time for that has passed.
May Allaah ta’ala protect us from ever entering the fire and envelop us in His Rahmah and may He make our best deeds our last and guide us until we enter Firdaws, Ameen. And give us patience over the remarks and foolishness of the disbelievers. Verily the promise of our Lord is true.

wassalam

Rabia Khan