Just to clarify the point I made earlier. Big Bang is not the point in time when the UNIVERSE stops working. It is defined as the point in time when our current THEORIES for the Universe stop working. I think it would be useful to keep this distinction in mind.

If anyone is interested, here is an answer from a theoretical physicist.

Hubble discovered that the Universe is expanding. Hence, as you go backwards in time, there are two options. Either all matter (and space) collapses to an extremely small region at some finite time in the past, or it does not. Hawking showed that it does. This is called the Big Bang. At this point in time, the description furnished by the theory of general relativity breaks down. The idea is that a better understanding of how general relativity and quantum mechanics work together will explain what happens at this point in time, and before. However, this work is ongoing and a final answer is still lacking. Ask me again in 50 years.