What happens if you try to draw a card but you can't?

You Burn Out. If your Main Deck is empty or has fewer cards than you need to draw, here's what happens (Rules 413.4, 431):

Draw as many as you can from your deck, then Burn Out: shuffle your Trash into your Main Deck (randomized), choose an opponent to gain 1 point, then finish drawing the remaining cards you needed (Rule 431.2).

Key details:

• If your deck is completely empty (0 cards) when you try to draw, you skip right to the Burn Out — you still draw the card after recycling your trash (Rule 315.4.b.1–2).
• If your Trash is also empty, you still Burn Out — your deck stays empty, you give the point, and if you need to draw again, you'll Burn Out repeatedly until an opponent reaches the victory score and wins (Rule 431.3).
• Points from Burn Out can end the game immediately — they don't require Conquering (FAQ #6736).
• Revealing or looking at cards from an empty or nearly-empty deck does not cause a Burn Out — that rule is exclusive to drawing and moving cards between zones (Rule 431.1.c).