For some reason the app didn't save my deck file (unless it ended up in some obscure folder) nor did it properly save my GIF (it's not animated) so I guess I wasted 3 hours of my life
If you attempted to save a .deck file then you should probably search your device for it. Searching for "*.deck" should give you a list of every decker file you have on your device. If you saved the file while your drawing was on screen it'll still be there when you open it again.
(If you find it you can open it with File > Open... here on the web upload, or in your own copy of Decker.)