How can I start on boot-level, when with the setup-keys a folder-with-question-mark shows up?

After my Mac mini (Late 2014), OS X Yosemite (10.10.5) worked slowly and one mouse-brand after another worked well for a while until again and again "nothing goes", then I started to erase the Mac mini HardDrive and also started to erase free space (as offered, because I requested it) - unfortunately in cleaning up my desk I touched several Mac mini key-board keys: the site changed to something else and I only could shut down without completing the erasing or new-setup:

NOW when starting with setup-keys or without setup keys a folder-with-a-questionmark shows up and I cannot continue (neither to erase again, nor to setup the Mac mini)!

What can I do now - in the Mac mini condition of a folder-with-a-questionmark showing up - to erase again or to start the setup and download OS X again?

jona li