This is about verb 'pyaas lagna'.
अचानक उसे मालूम हुआ कि बहुत देर से उसे प्यास नहीं लगी है*(NOT hain हैं)
"Suddenly she came to know that for a very long time she had NOT felt thirsty"
pyaas lagna - to feel thirst.
pyaas nahin lagna - to NOT feel thirst.
If I feel like drinking water, in hindi, I will say -
मुझे प्यास लगी है. Mujhe pyaas lagi hai i.e. I am thirsty or I am feeling thirsty
If I am not thirsty,
मुझे प्यास नहीं लगी है. Mujhe pyas NAHIN lagi hai - I am not thirsty
Similarly,
he/she is thirsty
उसे प्यास लगी है
he/she is NOT thirsty
उसे प्यास नहीं लगी है