(Zumar) 39 : 54
وَأَنِيبُوا إِلَىٰ رَبِّكُمْ وَأَسْلِمُوا لَهُ مِنْ قَبْلِ أَنْ يَأْتِيَكُمُ الْعَذَابُ ثُمَّ لَا تُنْصَرُونَ
And you turn into your Lord repentantly, and you submit all to Him before coming the punishment to you, then you will not be helped.
The punishment mentioned here is death. When death comes, there will be no opportunity for repentance, and they will not be helped. See explanation 2: 254; 13: 27-28; 63: 9-11.