Microsoft's game creation tool box has ended, after only being out for about a year.
Online servers have been shut down, and all user-created content is gone for good. If you purchased the game physically or digitally between the time it released (October 5, 2015) and May 12, 2016, you will receive credit on your Microsoft Store account.
My Two Cents
Project Spark was one of those sensations that generated a lot of buzz when it was announced, but upon release I never heard anything else about it. I wouldn't be surprised if the game had a pretty devoted community of creators constantly using it, but it's sad to me that this is the first time since it's release that I've seen Project Spark in the news. At least the game had a year long run, so anyone who wanted to check it out should have at least had the chance to.