To avoid attracting predators, mother rabbits only visit and nurse their offspring 2-3 times a day, so don't jump to conclusions if you've found a nest of uninjured baby rabbits with no motherly supervision in sight.
However, if you strongly believe the bunnies may be orphaned, cover the nest with natural materials (grass, leaves, etc) found around the nest. Then use sticks to mark an "X" over the nest. Check the nest after 24 hours. If the "X" has been moved, but the nest is still covered, that means the mother has returned. If the "X" is undisturbed, contact a wildlife rehabilitator.
Be sure to keep your pets away from the nesting area, and avoid touching the baby rabbits — unlike birds, foreign smells may lead to the mother abandoning her young.