What happens when you run out of cards
What happens when all the cards of your deck have been drawn in Hearthstone?
Any time that you're forced to draw an additional card, when there are none left to draw, your hero will experience Fatigue damage instead of getting a card.
Fatigue damage starts out at one and increases by one every time it triggers again, i.e. one damage the first time, two damage the second time, and so forth.
When cards get moved back to your stack after you have exhausted it once, you will not get damage for successfully drawing a card, but the Fatigue counter will not be reset.