Without the sun, humanity has lost.

The reason for that is simple: The sun powers everything. Without it, we don't get the heat to keep all those processes working that make Earth be the place it is.
First, the atmosphere would probably freeze or just become liquid. Then, when the Earth is in vacuum, the oceans wouldn't help anymore at all. This is because in vacuum, water simply doesn't want to exist in the liquid state - if it doesn't freeze it automatically starts to boil (even at, like, 1 degree Celsius).

Then, we wouldn't have anything anymore that could balance the temperature out. Furthermore, we would have to filter the air that we could preserve in underground bunkers or the like and at some point, we would run out of supplies. At this point, the normal folk would be long dead.

To conclude this, it would certainly be an interesting topic to write about, if it doesn't bother you that at the end of the book, everyone would be a really cold, frozen ball of frozen meat and blood. You know, frozen. It would certainly involve a lot of cold and death.