smash up? run out of cards
This is covered on page 4 of the rules.


  If you need to draw or reveal a card and your 
  deck is empty, shuffle your discard pile. Put it on 
  the table face down—that’s your new deck. Start 
  drawing from there.


If you have no discard pile as well, that would be a very odd game!  I've never seen a game go anywhere near that situation though.