If you have not already done so, have your veterinarian come out and examine the mare and foal, as soon as possible. Since the foal has likely never nursed the mare, it will likely not have gotten any colostrum and will need to get a plasma transfusion and be placed on antibiotics. Also, it may be quite dehydrated and have low blood sugar, which needs to be treated to get the foal interested in eating. Your veterinarian may be able to tranquilize the mare and help you get the mare to accept the foal. Maiden mares may not allow the foal to nurse due to a sore udder. Massaging, hot packing, and milking the mare may help her become more comfortable and allow the foal to nurse. Also, placing the mare in a stocks that gives the foal access to the udder, and possibly hobbling and twitching the mare, may help.
If you have a breeding farm in the area, see if someone from the farm can help you with the mare and foal. The foal needs to nurse at least every two hours.
If the foal needs to be raised as an orphan, it needs to initially receive 10 percent of its body weight in goat's milk or foal milk replacer over a 24-hour period (fed every two hours), preferably from a bucket to reduce the risk of aspiration pneumonia and bonding to humans. If the foal does well, slowly increase the milk to 20 percent of body weight over a couple of days. Once the foal is doing well, using an acidified milk replacer that can be fed cold and left out for a prolonged period without spoilage concerns will decrease your time commitment. If at any time the foal does not act normal and eat well, contact your veterinarian, as a foal can deteriorate quickly if it's not eating.