I bought a small cactus about five years ago, and it grew fine for the first few years, but now it's completely stopped. I water it rarely, like I was taught, the soil is sandy, and the window faces south. Maybe the pot is just too small for it, or is there something else I need to change?
It's probably not the pot - cacti actually prefer tight quarters. Check if it's been sitting in the same place for a few years without fresh soil. Over time the earth gets depleted and compacted, even in a pot that's big enough. Try repotting it into fresh cactus substrate (or mix in a lot of coarse sand and perlite into regular soil), and while you're at it, inspect the roots - they might have rotted or dried out completely. One more thing: even on a south-facing window sometimes there's not enough light, especially if the window is dirty or your cactus is standing behind other plants. Plus in winter there's obviously way less sun.
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