Sorry for my grammar, I just read my own question and so I wrote it in a confusing way. Let me try it again. After I made an "archive and install" I selected files I wanted from the newly made Previous System Folder. Then I sent that folder to trash and emptied it to finally try and gain back some space.It seemed as if it had worked, since now I had 20 GB of free space, but then I restarted the mac and that was when I got the flashing question mark. I tried again to do an "archive and install" but now the disk utility can´t find my hard drive.
I hope that was clearer. Thank you!
Once you were sure everything worked you should have deleted the (now redundant) Previous System Folder.
How to Archive & Install:
and this also:
BUT: Don't install older versions of Mac OS than what came with your computer: