In the popular imagination, the Big Bang is a great explosion; at one time there was nothing, then matter erupted into previously empty space. However, the Big Bang is the beginning of spacetime itself, not an event in time.
Asking about a time before the beginning of our spherical spacetime is like asking what lies north of the North Pole. There is no such thing.
There was a long history of speculation that in quantum gravity, unlike Einstein's classical theory, it might be possible for the topology of spacetime to change.
What lies north of the North Pole?
Contract law is essentially a defensive scorched-earth battleground where the constant question is, "if my business partner was possessed by a brain-eating monster from beyond spacetime tomorrow, what is the worst thing they could do to me?"