It's not recoverable unfortunately, seems either the firmware is broken or a hardware issue. The last time it worked I factory reset it to see if that would stabilise it, but it never booted again. I contacted Amazon about replacing it, but they won't do anything because it's not registered to my account (because it was removed when I reset it).

I was also running Projectivy [sic] Launcher, but it launched a minute after the stick started which wasn't a great UX so I ended up disabling it because it annoyed the family.

Either way, I'm not worried about recovering it now; it would have been nice to use it for a few thing, but when it stopped working I just went out and bought a new TV.