The only thing (usually) end-user-serviceable on these things is the filter screen up in the control head.
There is a cover that pops off, and after relieving pressure (you need a manual) you can unscrew a thing and pull out a screen that usually is caked with brown/black crud.
Somewhere on the control head (the thing on top of the thin tank) will be the make/model/serial # and with google you should be able to download a manual pdf.
If you haven't added salt in a year, the resin will be thoroughly depleted and may need a few regenerations in a row to get back to normal.
There is also a setting on most units that controls how much salt/brine is used each time, and your local municipal website/water dept. may publish the level of water hardness in your area, measured in grains per gallon or parts per million.
The harder the water, the more salt you need to soften it.
-K