A chicken with an impacted crop will not be passing much if any poo.
It is a blockage of the crop, and seeing as their digestive system is about 15 minutes long, if nothing is getting through, nothing is coming out either.
The hen may still lay for the first day or so, but will quickly go down hill and will probably stop.
The hen will be lethargic, often hunched over with shoulders high up (like a child in a bad mood) staring, looking vacant, not responding quickly to being touched/pecked by the others, and will often go to one corner standing still and generally looking really sad and depressed.
They often don't try to drink and definitely won't be eating - as its look constupation of the stomach.
It's important to notice and respond immediately. You will notice them getting worse in a couple of hours, they can be dead in 12-24hours of you noticing, sometimes even less if you don't watch them daily to check everyone seems alright.
Take the hen inside, in a warm, quiet place supply a bowl of warming water, you must encourage her to drink - even if this is just drips at a time.
Dehydration is the biggest and quickest killer!!