The Star is a card of hope and healing. Coming after the Tower, it promises that there is a future, and offers a light to guide the person on their way to that future. It is a drink of cool water (knowledge) in the wasteland, a promise of tomorrow. Reversed....
1) Opposite: Clearly, the opposite is that there is no future, no hope, no healing. Or, more to the point, that any hope or promise offered is going to be false hope. The star one is following is not fixed, and will lead the person astray. There is a feeling of being lost, with no way out.
This interpretation does not speak well for one who is sick or emotionally or psychologically hurt - they're not going to get well any time soon; they may even have taken a turn for the worst.
2) Blocked: We can think of this interpretation as there being clouds at night blocking out the star that one is following. Unlike the opposite, we know the star is there, but we can't see it. The person is feeling hopeless, but that doesn't mean there is no hope - they just can't see it.
I would read this as intense pessimism; the sort of cynical or depressed out-look that insists that the world is coming to an end, there is no future, the glass is half-empty, etc. The sick person feels that their illness will never end, that there is no cure.
This, however, is not accurate. There is hope and healing. But something is standing in the way of our querent finding it - either their own refusal to see it, or some other dark cloud.
3) Upside-down: Reversed, the waters of knowledge and healing that the Star pours out fall into the sky, and both pond and urns and earth are left dry. The Star is no longer in the sky, and so can't shed its light on the earth.
Very like the blocked interpretation, this is a time of darkness. Only in this instance, there really is no hidden promise. Everything has run dry. No one is offering the querent a future or hope for a future. This is a person in a very desperate situation. At its absolute worse, they are dying or suicidal - and with good reason. They are getting no help; they are alone with no guiding light, emotionally, psychologically and spiritually bereft. It is a very bleak and black time for them.
Curious about this card when upright? Go to our Tarot Card Meanings.