I need to know how to help a chicken that can't poop.

[Hen Pen Jem]

I have raised chickens on river sand and dirt runs with no problems. Chickens spend their time by eating and searching for the right size particles to eat for grinding their food. Just make sure there is a variety of different size stone particles for them to search through. Most of the time constipation occurs when there isn't sufficient stones in the gizzard to grind unprocessed grains.

What is the difference between sand and grit?

Gail Demerow, The Chicken Encyclopedia, writes:
" grit\ Small pieces of a hard substance eaten by a chicken to aid digestion."
She also writes -
"Chickens that have access to scratching in dirt generally pick up all the natural grit they need"
Also -
"Chickens that eat only processed feeds - such as chick starter or layer pellets - don't need grit because saliva alone is sufficient to dissolve these feeds."

The question is did the sand cause the chick's constipation? Who knows? Their digestive tract is designed to work with tiny stones. People that respond here feel for you and want to help you, with varying opinions and advice.
In the end you will know what is best for them.
Continue to educate yourself, read Gail Demerow's books, she is an authority on chicken rearing.