If an opportunity, plan, idea, way of thinking, or pattern of behaviour goes 'out the window,' it stops or is lost completely, usually as a result of something negative. For example, if you had spent months planning and preparing to climb a mountain but broke your leg at the very start, then it would no longer be possible to continue with that plan. It would 'go out the window.' The idea behind the informal expression is that if you were to throw something out of a window, like that of a moving car, you would not be able to find the object again. It is common in both a social and professional context, and is similar to the phrase '(go) down the tubes.'