Rather than talk about what consciousness is, I'll tell you there are two necessary things for consciousness to arise:
wakefulness and awareness.
It goes like this:
wakefulness --> awareness --> consciousness
Which means that you need to be awake to be aware and you need to be aware in order to be conscious.
If someone has a disorder affecting wakefulness, such as being in a coma, then he/she can't be aware and therefore cannot be conscious. If someone is awake but is in a vegetative state, he/she can't have awareness and therefore cannot be conscious either.