〜たら is used in an if/when situation. For example, 高かったら、買わない means that if it's expensive, I won't buy it.
〜なければ is a negative conditional statement. For example, 彼でなければ、問題が解けない means that no one can solve the problem without him. Or, if it's not him, then the problem can't be solved. I think that it has a similar meaning with 〜なかったら.