When you fight a creature and it drains a couple levels off of you, do they ever come back? What happens if they drain to down to level 0?

Getting drained to level zero will kill your character. You can use either one of the clerical restoration spells or visit a temple store to restore your drained levels.

they never come back without the use of magic. Cleric's Lesser Restoration or Restoration can redeem those lost levels for your characters, or you can go to temples and the like and pay money for the priests there to restore you.

The only way to regain your lost levels is to use a restoration scroll or spell. Either of which can be bought at the nearest temple. Should you get drained to level 0 you die, that's that.

When you're level drained you can still earn experience, but you won't get the effect (rising in levels) until you've restored the lost levels. There are a few items that protect you from being drained and a priest spell (but that only lasts 5 rounds).

man, that sucks. I ran into a couple vampires early on down at the docks and they got a couple levels off of one of my characters. I am only a couple days into the game.

You can go to any of the temples within Athkatla and purchase a Lesser Restoration spell to cure the character, or if you have a Cleric of sufficient level you could have them cast it on him.

I just got round it a slightly different way with an NPC. Korgan was L12 and got drained to L4 by Bodhi. So I killed him and used the rod of resurrection to bring him back to life and full health.