In the text, it alludes to a strange fever that just takes over his body, rendering him weak, and eventually takes his life:
"His eyes blazed at her for one strange and senseless second...Then he fell with a crash to the ground...he was unconscious, and unconscious he remained, with some brain-fever...The third day of the illness was critical...He neither slept nor regained consciousness."
So, he collapsed, probably from exhaustion and fever, born from his obsessive hunt for "luck." Throughout the story, he is so obsessed with making money that he stays up late in the night riding his horse, hoping for a revelation that will win his mother some more money. So imagine a small child who gets very little sleep, unusual amounts of exercise, is fixated and obsessed with a certain life mission, and is zealous and intense in all that he does. I would imagine that he slowly weakened himself, from not taking proper care of his body. Kids need a lot of sleep, and studies have proven that stress undermines the immune system, making your body much, much weaker. The descriptions of Paul show him becoming more and more feverish underneath his obsession; Lawrence describes his "bright eyes" over and over, hinting at a fever that burns underneath his skin, born from his intense pursuit of luck. Eventually, it just wore him out, and his body was too weak to live anymore. Lawrence is making a grave statement about people's obsession with money; he hints that it is not healthy, and only takes from people without giving in return. Greed, materialism, and an unhealthy focus on the pursuit of wealth only sucks the life out of living, instead of making it better. In this story, Paul was the sacrifice that greed asked.
I hope that those thoughs help a bit; good luck!
THank u. It really helps