Rosemary is pretty hardy for your area and this past winter was not a very bad one. I suspect the problem is rot from wet soil conditions. That plant prefers a very well drained soil and not being water or rained on continually for long periods of time. Otherwise rot diseases get the upper hand and can cause rapid decline and death. If they are not completely dead try cutting back the brown stems and then wait and hope for the best. If they are in an area prone to soggy soil dig and move them. Should you decide to purchase and plant more rosemary plants make sure they get full sun and build a raised planting bed for them.
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