Pulling together all facts and hints from texts, it becomes clear that the sun still exists, but must be hidden somehow, so that only enough of its rays reach Pagan, that the world won't die, putting it into eternal twilight. The biggest hint comes from the description of Stellos task is "Assisting the Air":
"...Farther and farther he went, facing tremendous winds. There he finally found a shimmering cloud at the very verge of the sky itself and something he later called the ethereal void. As he entered the cloud, the light of day confronted him; he was the first and, to this day, only Pagan to see sunlight in many generations..."
This gives the final conclusions:
- The Sun is still there.
- A sort of cloud hides it.
- The Titans themselves seem to have no problem with it, meaning that they created this cloud in the first place.
It is unknown what happened after the destruction of the Titans, but most likely the cloud dissolved with them gone, returning the sun to the sky.