There is one word that describes the feeling of not caring about anything. It's "anhedonia." Even if you haven't heard of it, you can learn about it fast--AND get a plan to help you start caring again.
When it comes to talking about mental health issues online, why don't more people talk about depression? A huge percentage of the population deals with it, so why is it not more common? This might be why.
When you're feeling depressed and you're at your lowest point, it can be almost impossible to escape your confining vocabulary. But it can help. This is why it's worth a try. Here are other words for "depressed."