I thought maybe looking into how people in comas experience their time, but apparently it's common for them to have experienced much of what happened around them as a hazy dream[1], which means a lot of sensory input was still being processed. I'm not sure what it would be like to actually lose all sensory input. There's probably some disease that causes this, so maybe I'll stumble on it eventually, but even then we'll only get answers at this point if it's reversible...

1: https://www.quora.com/What-was-it-like-for-you-to-awaken-aft...