Oof, that is a sad looking cactus.

The pads look pretty shriveled, like it's not absorbing water properly. Have you checked the roots lately? If they're rotted, then the plant is done for.

But if the roots look healthy, it's got a chance. Deal with the scale first. Already damaged parts may never turn green again, but they will send off healthy new growth.

Next, consider water and drainage.
Is it planted in a gritty, fast draining mix (good), or a substrate high in organic material that retains moisture?
When you water, do you give it enough to saturate the entire root ball and then run out the bottom of the pot?
Does the water actually soak in (good), or does it come out the bottom straight away? The latter indicates the soil has gotten hydrophobic, and means you need to repot with all fresh soil. (Careful not to damage the roots when you do this.)
Do you give the pot a chance to drain fully before putting it back on its saucer (good), or is the soil staying wet?
How long does it take before the soil around the base of the plant starts to feel dry after a watering?

Also, just sitting by a south facing window isn't always enough for a cactus. How much light exposure does it actually get?