This is normally because the printer is applying an update that it had downloaded. It is imperative that you do not remove power from your Simple during this process. When the Simple turns back on you are good to start printing again.
However if, after an hour the screen is still black, this may be caused by a corruption on the Micro SD card in the Hub. Please follow these steps to resolve this issue.
1. Download and unzip the following file https://www.dropbox.com/s/11njc7qrfz027ol/Simple%20Pro.zip?dl=0
2. Undo the 4 screws that hold the front panel to the printer. There are 2 each side.
3. When the panel has been unscrewed, CAREFULLY move it forwards. There is a ribbon cable connecting the screen to the Printrboard and we dont want to pull on it.
4. On the underside of the panel is the Hub with a MicroSD card, remove the card and, normally using an adapter, place it into the computer that you have stored the files from Step 1.
5. DO NOT DELETE any of the files on the card and do not be tempted to format the card. There are unique files on there that this process will not replicate!!!
6. Drag and drop the files from the zip file onto the card. When it prompts you to overwrite, select yes.
7. When the copy process has finished, unmount the card from the computer and replace in the Hub.
8. replace the panel on the printer and power on.
9. Start printing!
You will find that any projects you had moved to the printer have gone, but you can just send these down from the cloud again.