If the targeted permanent or player is an illegal target by the time Explosive Revelation resolves, the spell doesn’t resolve. You won’t reveal any cards from your library.

If all the cards in your library are land cards, you’ll reveal all of them, then arrange them in your library in whatever order you like. No damage is dealt.

Once you start revealing cards from your library, it’s too late for players to respond. If you target a creature, for example, its controller can’t wait to see how much damage will be dealt to it before casting a damage prevention spell, activating a regeneration ability, sacrificing it for some benefit, or anything else. Any such actions must be done before Explosive Revelation starts to resolve.