Did you ever check your water softener salt level and it looks like it never goes down? Or does your water softener run but does not soften the water? If so, then you may have a problem known as a salt bridge.
A salt bridge is a hard over-crusting of the salt in your brine tank covering or spanning over the water. There may be a normal looking layer of salt pellets on top of the bridge which is hollow below.
A salt bridge can often caused by high humidity or from using the incorrect type of salt.
When a water softener stops conditioning water the problem is that something is stopping your water softener from regenerating. The regeneration cycle of the resin bed is where the water conditioning occurs.
Assuming you are not out of salt the problem may be one of a few possible problems. There could be a salt bridge that has occurred, or a crusted salt plug or other blockage between the brine tank and resin tank, or there may be a problem with the control valve (if an automatic water softener). Any of these problems may be stopping the unit from going through a regeneration cycle.
The most common problem however may be the salt bridge. A good source on how to repair this problem is found at this page from the Canadian Salt Company Limited.