A patch of moss gone brown and crisp looks finished, and people dig it out or throw it away. Most of the time that is a mistake, because moss does not die when it dries; it shuts down and waits, and a good soaking brings it back.
Why brown moss is usually just dormant
Moss has the ability to let its water content fall to almost nothing, going dormant in a desiccated state and resuming life when moisture returns. In that state it looks dead, brown and brittle, but the cells are intact and merely paused. This is how it survives drought on a wall or a roof, and it is why a summer-scorched patch greens up again after autumn rain. The default assumption with brown moss should be that it is resting.
Bringing it back
Rehydration is simple. Give the moss a thorough wetting with rainwater or low-mineral water and keep it damp, by misting daily or, for a detached piece, by soaking it in a bowl for a quarter of an hour. Within a day or two living moss starts to swell, soften and green from the tips. Set it somewhere shaded and humid while it recovers rather than back in the sun that dried it, and be patient over a week or so; a badly desiccated cushion can be slow to wake fully.
When it really is dead
Moss does have limits. Prolonged drought, baking heat, being smothered under wet leaves until it rots, or a dose of mosskiller will genuinely kill it, and dead moss stays brown, goes slimy or simply crumbles away without ever greening after repeated wetting. If a fortnight of consistent moisture brings no flush of green anywhere in the patch, accept that it has gone and start fresh, as in the growing guide.
Reviving moss bought dried
Dried sphagnum and some dried sheet mosses sold for crafts and orchids can sometimes be coaxed back to life, since fragments may still be viable, but it is hit and miss; preserved moss, treated with glycerine, is dead for good and will never grow. If you want living moss, start with living moss or fresh fragments rather than counting on reviving a bag of the dried stuff.