Normally the colon contains air and stool, both of which expand the inside of the colon. A collapsed colon means that there is almost no air or stool in the large intestine.
When a doctor sees a collapsed colon on an X-ray or CT scan, the first thought is the presence of a small bowel obstruction. If a patient with an X-ray showing a collapsed colon was having abdominal pain, small bowel obstruction would be the likely diagnosis. A small bowel obstruction usually feels crampy, with waves of pain that can be very intense.
In addition to the collapsed colon, the doctor would determine if the pictures show an expanded (dilated) small bowel. This would almost surely confirm the diagnosis of a small bowel obstruction.
When there is a small bowel obstruction, the intestines keep contracting. The intestines sense that something is inside the intestine and needs to be moved along. The intestines don't know that nothing can move beyond the blockage.
At the onset of obstruction, the small bowel in front of the obstruction will begin to dilate. Soon after, the intestinal contractions will push out all the contents below the blockage. Once everything in the colon is out, the colon will collapse. The person will stop passing any stool or gas from the rectum.