My Russian Sage is turning brown and is no longer blooming. It is less than a year old and looked beautiful until 2 weeks ago. Other surrounding plants are not affected.
Sage, like lavender does not like "wet feet". If you have had lots of rain or been watering a lot, the plant may be suffering from root rot. I would cut it back by 1/3 and wait. (Don't cut back past all living leaves) If your soil is heavy clay, dig the plant up in early fall and work in some organic matter and coarse sand to improve drainage. Winter kill from rotting roots is a bigger hazard than cold temperatures. https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm