A softener that stops working does not always fail in the same way. Sometimes hardness returns overnight, and sometimes it creeps back over several weeks. The pattern at your taps tells you which kind of problem you may have, and it tells you what to check first, before anyone opens the tank.
Two ways softening stops
The first is a bypass. A valve on the plumbing sends water around the softener tank, so untreated water reaches the fixtures. This usually happens suddenly, often after plumbing work, a filter change, or someone turning valves to stop a leak and forgetting to turn them back. The second is an exhausted softener. The treatment material gradually loses its ability to soften water, so hardness returns slowly, and the change may first show up as more soap use or more film on fixtures.
Check the bypass position
Look at the valve assembly on the plumbing above or beside the tank. Most systems have a handle or lever with labeled positions such as service, bypass, and off, though designs vary. Read the label on the valve body rather than assuming what the handle means. If the handle sits in the bypass position, or if it is stiff and clearly left partway between positions, you have found a likely cause. Do not force a stiff valve. Note its position, take a photo, and call a technician if you are not sure how it should sit.
Check the pattern at the taps
In a whole-house system, a bypassed tank affects every tap downstream of it at once. If only some fixtures show hardness, the home may have a second line that runs outside the softener, or a tap fed from a different point in the plumbing. Compare a few taps in one sitting. A uniform change points to the valve. A patchy change points to the plumbing layout, and that is worth mapping before you blame the equipment.
Check the control head
Confirm the unit has power and that the display shows a normal time and status rather than an error or a blank screen. Note any error code and look it up in the manufacturer manual that came with the unit. A blank display on a unit that has been running fine often means a power or outlet issue rather than a softening problem.
Signs of a slow decline
If the softener has been running for years and the change came on gradually, the likely cause is the treatment material wearing out or fouling. You will usually notice a longer trend: more detergent per load, more film after each use, and a lather test that gets worse over time. Keep a simple written log of what you observe. A pattern over weeks is far more useful to a technician than a single bad morning.
When to stop and call
Call a technician if the valve is stiff, if you cannot tell which position is correct, if there is any leak or wet floor, or if the display and the taps disagree in ways you cannot explain. Bring your notes on which taps were affected, what the valve position was, and how the change came on. Those details will shorten the visit.