The Quran

فَاعبُدوا ما شِئتُم مِن دونِهِ ۗ قُل إِنَّ الخاسِرينَ الَّذينَ خَسِروا أَنفُسَهُم وَأَهليهِم يَومَ القِيامَةِ ۗ أَلا ذٰلِكَ هُوَ الخُسرانُ المُبينُ

Muhammad Asad

and [it is up to you, O sinners, to] worship whatever you please instead of Him!" Say: "Behold, the [true] losers will be they who shall have lost their own selves and their kith and kin on Resurrection Day:¹ for is not this, this, the [most] obvious loss?

Implying that on Resurrection Day they will be irretrievably separated from all whom they had loved, and all who had been close to them in this world. The "loss of one's own self" signifies, I think, the destruction of one's true identity and uniqueness as a human being, which is described in the next clause as "the most obvious loss" that man may be made to suffer in the life to come.