take a bucket a really warm water(not hot) and towels to the barn. Soak the towels in the really warm water and wring them out, use them as warm wet compresses on the udder. Hold the warm wet towels on the udder , keep repeating, don't let the water cool down too much. and be careful not to burn the mare. Water must be warm but safe. Many times the warm wet towels will bring the milk in. Have you tried milking her at all? Sometimes that will stimulate milk production.
Good luck!
Robin
PS... I have a sick foal at home. He is a couple days old. We have been giving him Seramune (orally) Its IGG in a bottle, and can be given po, ask the vet about this, ok, maybe it would help???