You've not given much information to help you, but have you asked your vet? regional BHS? Local breeders (found via google) or you could try the mare and foal sanctuary? http://www.mareandfoal.org/html/index.php
Johanna Vardon National Foaling Back is a wonderful place to start she will help you. http://www.nationalfoalingbank.com/ She will tell you about how to help you mare. Good luck I hope out of this sad loss you have the joy of helping someone else out.
Please can you give us an up date at some point. People always ask questions and I guess I am nosey but I would love to know what happen next. Thank you