Either of two things:

Nothing. However, I don’t think most people quite grasp the meaning of that. Kind of how they think that before the big bang there was just empty space. No, empty space is not nothing. There’s no empty space, there’s no time, there’s nothing. By definition it cannot be experienced. Experience simply ends. It’s as if nothing ever happened. The universe could just as well have never existed.

The more optimistic theory is that consciousness is in a way immortal. You can only experience being, not not-being. It’s kind of how when you go under general anesthesia and then wake up it’s quite unlike sleeping. When you’ve slept you have the sense of time having passed in between. With general anesthesia this is not the case. One moment you feel sleepy and then you wake up in another room. From your subjective experience you never lost consciousness to begin with. Whose to say that something similar doesn’t happen with death. Instead of experience ending it just moves elsewhere. It’s a pretty difficult concept to explain but it’s somewhat similar to the idea of quantum immortality.