Then We exchanged in place of the bad [condition], good, until they increased [and prospered] and said, "Our fathers [also] were touched with hardship and ease."1 So We seized them suddenly while they did not perceive.2
Translation: Saheeh International (English)
1 Instead of being grateful to Allāh for His blessings, they merely attributed them to the changing fortunes of time.
After trying them with suffering and illness Allah improved their situation and turned their suffering into well-being and safety, until they became many and their wealth increased, and they said that the bad and good that they had experienced was a normal part of life and that the same thing had happened to their ancestors before them. They did not recognise that there was a lesson to be learned in the difficulties that occurred to them and that there was a test for them in the good that they were given. Then Allah overtook them suddenly with the punishment, while they were unaware of it and did not expect it.