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They like a pH of between 6-7.

If your plants have been healthy for several years, then suddenly start browning, something in their environment has changed. Pay attention to possible changes in light, water, and soil. Also, is it possible they were affected by cold winter weather? This article should help you: https://www.gardeningknowhow.com/edible/herbs/rosemary/brown-rosemary-plants.htm

Root rot is a generic term describing an effect -- rotting roots -- caused by any one of a number of pathogens, usually a fungus of some kind, but also possibly bacteria or even viral infection. Just as you may get infected by a "bug," so can your plants. Plants have many ways of fighting infection, but they need to be strong and healthy to do so. Heavy clay soil is not the best for rosemary; it likes good drainage for the long haul. If you want to save your plants, you could try digging them up and replanting elsewhere in a soil amended with sand and organic material. Here's one article that will get you started: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm

It may be dead. Cold weather will kill rosemary. If any of the stems are green, it may come back. This article will help you determine if the stems are still green: https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm

I would definitely hold off on any pruning while the plant is in bloom. For more help, this article should help: https://www.gardeningknowhow.com/edible/herbs/rosemary/pruning-rosemary.htm

This article should help with that: https://www.gardeningknowhow.com/edible/herbs/rosemary/harvesting-drying-rosemary.htm