At least with the Brother printers I've owned (and some of which I still do), they actually shine a light through a small bit of clear plastic in the toner cart

At least with the Brother printers I've owned (and some of which I still do), they actually shine a light through a small bit of clear plastic in the toner cartridge. Taking that cartridge out, giving it a good shake (outside, no need for adding potentially cancer-inducing toner dust in your room), and putting it back in will make the printer work again.