I'd imagine you could have a Boolean that starts as false, let's call it FallThrough. On your code that makes the player fall and lose, FallThrough has to be true in order to fall and lose. That way, when the game starts, they don't immediately fall and lose.
When your player first presses the up arrow and FallThrough is false, set FallThrough to true.
If you use a global variable, make sure you reset FallThrough to false when the player clicks restart.