This verse takes us back to the early verses which talk about the Last Day and its horrors:

 الْحَاقَّةُ (1) مَا الْحَاقَّةُ (2) وَمَا أَدْرَاكَ مَا الْحَاقَّةُ

(The Inevitable Reality (1) What is the Inevitable Reality? (2) And what can make you know what is the Inevitable Reality?) (Al-Haqqa: 1-3)

Then the next verses talked about the worldly punishment which infected the people who rejected this Day. Not to mention the inescapable punishment in the hereafter, which they used to deny it. In the Day of Judgment, the obvious reality will surprise them though they were warned against it by the Messengers.

(… فَإِذَا نُفِخَ فِي الصُّورِ ) (Then when the Horn is blown…): The word “when” expresses the suddenness. Blowing the horn is a sign of the beginning of an event. After the first Blowing of the Horn, all living creatures will die. After the second Blowing, all the dead will raise alive. This last Blowing is a call for all the creatures to come out of their graves. In this regard, Allah (swt) says:

 يَوْمَ يَدْعُوكُمْ فَتَسْتَجِيبُونَ بِحَمْدِهِ وَتَظُنُّونَ إِن لَّبِثْتُمْ إِلَّا قَلِيلً

(On the Day He will call you and you will respond with praise of Him and think that you had not remained [in the world] except for a little.”) (Al-Israa: 52)

When Allah (swt) calls them from graves by the last Blowing of the Horn, they will get up with obedience and submission. No one will be arrogant or lazy. That was in the worldly life, not in the Hereafter.

(… فَإِذَا نُفِخَ) (Then when the Horn is blown …): This is the last Blowing of the Horn which is the blow of resurrection. In this regard, Allah (swt) says:

وَنُفِخَ فِي الصُّورِ فَصَعِقَ مَن فِي السَّمَاوَاتِ وَمَن فِي الْأَرْضِ إِلَّا مَن شَاءَ اللَّهُ ۖ ثُمَّ نُفِخَ فِيهِ أُخْرَىٰ فَإِذَا هُمْ قِيَامٌ يَنظُرُونَ 

(And the Horn will be blown, and whoever is in the heavens and whoever is on the earth will fall dead except whom Allah wills. Then it will be blown again, and at once they will be standing, looking on.) (Az-Zumar: 68)

(… فِي الصُّورِ… ) (…the Horn …): The Horn will be blown two times and the Blower is the Angel Israfil. The Prophet (saws) said:

“When Allah (swt) has created the heavens and the earth, He has created (As-Sur) and given it to Israfil. Since then, Israfil is putting it on his mouth, looking at the Throne of Allah and waiting for the command”.

Abo Hurayra asked the Prophet (saws): “What is The As-Sur?” The Prophet said: “The Horn”

Abo Hurayra asked: “How does it look”?

The Prophet (saws) said:

“It is great, by the One Who my soul is in His Hands, each of its circle is as wide as the heavens and the earth. Allah will order Israfil to blow in the horn three times; the first is the blow of horror, the second is the blow of death, and the third is the blow of Resurrection.”

So, when the disbelievers are taken by surprise with the blow of the Horn and the Hour in which they were disbelieving, they will be overwhelmed by astonishment. They will become speechless out of surprise when they see what they disbelieved in happening before their eyes.