Discus can be very hard to keep, they are very sensitive to the water they live in mainly the Salinity level of the water. Freshwater fish drink little to no water they obtain the water they need thru Osmosis (they obsorb the water they need thru there body). If the Salinity of the water is higher than there body the fish will not get the water it needs to survive this is called Osmotic Stress, symptoms are loss of color and will quit eating , may dart, fins will begin to look frayed. (hard to reverse depending on how advanced the cell destruction is within the fish).
Water parametors for Discus is Acidic, softwater ( lower carbonate and bicarbonate within the water).
A second reason for this with Discus is that the fish was suddenly frightened will show basicly the same symptoms, will hide and stop eating.
Hard to say without seeing the fish and what the water parametors are , pH and the TDS level (gh & kh) and what types of fish are being housed with the discus.