My kindle is fully charged.
I tried short-pressing the power button, which did nothing.
I tried long pressing the power button, which made the green LED flash between green and amber.
I then tried holding the power button down for quite a length of time (not sure how long), which resulted in the screen turning blank then returning back to the picture of the tree.
I've tried looking every kindle help page I could find and I still have no idea what to do.