Yashichiro Takahashi’s Shakugan no Shana light novel series ended back in 2011. However, it got a revival of sorts within the pages of Dengeki Bunko Magazine’s September edition. This is because that very issue contained a brand new Shakugan no Shana story.
The main novels may have ended in 2011, but Takahashi wrote several side stories afterwards. He then stopped, but this new story would then go on to be the first story in the franchise for four years. This one also comes about six years since the main story ended. However, there’s still no word whether this new story will be getting a follow-up.
As for the TV anime, its final season began on the same year as the release of the final novel, which is 2011. It then ended with its final season, Shakugan no Shana III (Final) in 2012. The story follows Shana, who must must protect the world from the denizens of the Crimson Realm, which devour the existence of humans.