Spore 2D (lost Flash game; 2009)
Spore 2D was a flash game based off the 2008 simulation video game, Spore. It was created as attempt to promote the Wii release of the game, Spore Hero. In the "training" section of the game, players could "train" their creature by forcing it to deflect objects shot from totem poles. The more the creature leveled up, the more parts the player would unlock. Players could also drag user-created creatures from the Sporepedia, to be rendered in 2D form.
The game became lost sometime in 2012. All that exists of the game is a couple of gameplay videos, as well as the SWF file, which can be found on the Internet Archive. However, the SWF file is missing a critical XML file, rendering the game useless, as it's stuck on the loading screen.
Since Spore 2D is a flash game, theoretically, it could be recovered by getting the .SFW file (presumably from The Wayback Machine), and opening it in a Flash Player. Unfortunately, the game is still missing other files, such as the aforementioned XML.
Thankfully, there are free .SWF decompilers that exist online, such as JPEXS Free Flash Decompiler, that can help us retrieve the URL to the hidden XML file. Sadly, the link to the Spore 2D XML file is not available online, and the only reference to it on the Wayback machine is a domain name registrar page.