Word by Word Analysis

فَلَمَّا نَسُوا۟ مَا ذُكِّرُوا۟ بِهِۦ فَتَحْنَا عَلَيْهِمْ أَبْوَٰبَ كُلِّ شَىْءٍ حَتَّىٰٓ إِذَا فَرِحُوا۟ بِمَآ أُوتُوٓا۟ أَخَذْنَـٰهُم بَغْتَةً فَإِذَا هُم مُّبْلِسُونَ 44

So when they forgot that by which they had been reminded, We opened to them the doors of every [good] thing until, when they rejoiced in that which they were given, We seized them suddenly, and they were [then] in despair.