Fictional comatose patients in fictional hospital emergency rooms often just appear to be sleeping, maybe with a nasal cannula, an IV and attentive family holding a bedside vigil. And that's a fairly standard representation across the board for TV medical dramas.
In reality, not all comas are the same. They're classified based on a patient's level of eye response, verbal response and motor response. The lower the score, the more severe the coma. The causes of a comatose state can be extremely varied, but traumatic brain injury (such as a concussion or lack of oxygen from drowning) or certain conditions such as diabetes are the most common.
Comatose patients are often hooked up to tubes, pumps and machines to help keep the body functioning. Treating a coma may require ventilation, a feeding tube, a catheter for bladder control, a catheter to monitor blood pressure and a heart monitor, among other lifesaving devices and medications. Most of that equipment isn't especially telegenic, and some of it would block an actor's face, so TV dramas tend to skip it.