I came home to my Mac Book Pro (late 2009) showing me the blinking question mark folder.
I followed all the instructions available online (Reset NVRAM, SMC) but neither boot screen (pressing Alt during/before start-up) nor Internet recovery (Command+ r during/before start-up) shows me anything. They just lead to an empty grey startup screen.
I attached an external hard drive with bootable macOS that I have used on this computer before, but it changes nothing - boot screen doesn't show.
EDIT: I solved the Problem by exchanging the IDE Cable. I do not know how this has effected the start-up process, but now i was able to boot from the external drive and from my drive as well.