Device not booting past the tree picture.

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.