What happens after Mr. White wishes for 200 pounds in "The Monkey's Paw"?

Immediately after Mr. White makes the wish for two hundred pounds, three things happen:

A fine crash from the piano greeted the words, interrupted by a shuddering cry from the old man. His wife and son ran toward him.

His skeptical and fun-loving son Herbert plays a loud chord on the piano, as if to create an effect of something like a magical feat being performed. His father drops the monkey's paw because, as he tells his wife and son, it seemed to twist in his hand like a snake as he made his wish. Throughout the story, it is never clear whether the wishes are being granted by the monkey's paw or they only appear to be granted as a result of coincidence. As far as the paw seeming to twist in Mr. White's hand is concerned, that may have been caused by his fright when Herbert startled him with the "fine crash" from the piano, which was more or less what Herbert intended.

After that, nothing happens. The three family members stay up a little while longer and then all go to bed. That is the end of Part I.

Mr. White is obviously a modest man. If he had wished for something grander, such as a mansion or a big bag full of gold coins, then the reader would feel sure the monkey's paw had supernatural powers. Mr. White only wishes for two hundred pounds, though. It could have been sheer coincidence that Herbert would get killed in a factory accident the next day and the company would decide to give Mr. and Mrs. White the sum of two hundred pounds as compensation for losing their son.

What occurred right after Mr. White wished for 200 pounds in "The Monkey's Paw"?

Three things happen immediately after Mr. White makes his wish for two hundred pounds.

A fine crash from the piano greeted the words, interrupted by a shuddering cry from the old man. His wife and son ran toward him.

"It moved," he cried, with a glance of disgust at the object as it lay on the floor. "As I wished, it twisted in my hand like a snake."

Herbert had seated himself at the piano just before his father made the wish. Herbert is supremely skeptical about the supposed powers of the monkey's paw and obviously intends to make a joke of it. When his father says, "I wish for two hundred pounds," Herbert does his best to play an ominous chord on the piano as a suggestion that something supernatural is transpiring or is about to transpire.

These happenings prove nothing either way. And this is author's intention. He wants to leave the reader in doubt as to whether the monkey's paw had magical powers or whether it was nothing but a mummified relic. Mr. White may have only thought that the paw twisted in his hand. If Herbert had not played that crashing chord on the piano, his father might not have imagined that the paw moved. On the other hand, maybe the paw does have magical powers and maybe it will move any time its possessor makes a wish.

In the beginning of Part 2 of the story, Mr. White brings up what will be a question with everything that subsequently happens.

"Morris said the things happened so naturally," said' his father, "that you might if you so wished attribute it to coincidence."

This  sentence is of prime importance to the story. The reader will never know for sure whether the strange events occurred because of the supernatural power of the monkey's paw or whether they were all coincidences. It was not a coincidence that Herbert got killed by a machine. That could have happened to anybody who worked around machinery. What was a seeming coincidence was that the company paid two hundred pounds to the Whites as compensation for their loss of their son. 

At the horrifying climax to the story, Mr. White uses his last wish to make the knocking cease and for the person knocking to go away. The knocking stops immediately, and there is no one out there when Mrs. White finally manages to open the door. But the reader will never know whether it was the mangled and decaying Herbert doing the knocking or whether it was some harmless stranger who was lost and trying to get directions. It seems unlikely that a stranger would show up at just that time--but it seems equally unlikely that Herbert could climb out of his grave and come back home to live with his parents.