Nathan's 70% number is close but I'd actually argue it's way higher than that. I've been doing this for a while and dirty condenser coils are the culprit probably 85% of the time on basic no-cool calls. The door switch and seal thing sandra mentioned is real, but that's more of a freezer issue than a fridge issue in my experience. A bad seal won't cause a total no-cool, it'll just make the compressor run longer and waste energy. Coils are literally step one every time, people just skip checking them because they think it's gotta be something harder.
My old shop tracked this, and a dirty condenser coil was the top cause for about 70% of basic no-cool calls. It reminds me of what @sandra715 said about door switches, where the most common fix is often the simplest thing to overlook. You see that pattern everywhere, where the obvious answer gets ignored while people look for a harder one.