You have about a month to re-mate her. This takes coordinated management, especially if she must be transported to the stallion or you have to get shipped semen. Your veterinarian can use hormones to return non-pregnant mares to their estrous cycle if necessary. They become pregnant as readily as younger mares. But they don't stay pregnant.
Older mares have a high incidence of early embryonic deaths.
Owners of older mares are usually eager for a foal from these mares because they often have a record of producing winning foals. Many mares foal in April and May, so now is the time you need to know the pregnancy status of each broodmare. It saves time and reassures you that pregnant mares are safely past the first critical 40 days of pregnancy. You can now concentrate your management on open mares that must be re-mated. Now that we're in early summer, consider reimplanting your calves to increase market weight and returns at marketing.