Ending A Series: How To Handle Cancellation
Once upon a time, having a publisher decline to pick up more books in a series you'd written was enough to cause keening and tearing of attire over the death of your series. Now, however, when the popular guy takes you to homecoming but dumps you before prom, you can take yourself to the damn dance. Sure, the quart of Haagen Dazs still gets devoured and there are still sniffles, but the pajamas escape any serious mauling and you don't have to worry about complementing his cummerbund.
Okay, okay, I'm done with quirky analogies and bad metaphors.
In all seriousness, it's never fun being told your work isn't wanted anymore. Twice more when that means you're not getting paid for future work. Thrice more because you have to re-enter the submissions arena.
If your heart is still deeply in that world and you have more stories you want to tell in that world, then keep writing the books and self-publish them. Remember, readers who found your earlier works will look to retailers for more of your books...not publishers (much to publishers' consternation). Keep your website current, keep your readers informed of release dates.
Readers care more about the author than the publisher.
As long as there are no legal limitations to you pushing forward, no one can stop you from publishing your work. Double check your publisher contracts. If you have an agent, this is the time to give them a call. You may have retained rights to the world, but the publisher may have placed restriction on the use of your author-name. Commonly, publishers will specify an amount of time that must pass before the name you used with them can release a book with another publisher/independently. The timing might work out well or it might mean you have a bit of a delay before pushing the next release live. However, if there are no contractual limitations, go forth and write more books.
The publisher gave you an advance (maybe) and helped readers find you.
Maintaining and growing your audience is always your responsibility, not the publisher's.
While you're faceplanted in the Midnight Cookies & Cream bemoaning your changing fate, take a few snarfs to consider your newly found freedom from contracts. Maybe it's time to write that vampire amoeba superhero book about which you've been daydreaming. Or kick-out a few novellas told from a tertiary characters POV from the cancelled series...just to keep the readers interested while you sort yourself and the future of the series.
A publisher dropping your contract or declining to pick up more books in the series doesn't have to be the end of the series. This isn't TV. Cancellation doesn't mean the End.
A book series ends when you, the author, decide to end it.