Reinstallation of OS after erasing hard drive - for resale

I misunderstood the Apple Support instructions for erasing the hard drive of my Macbook Air mid-2013 in order to leave it in an "out-of-box" state. I pressed Command-Q too soon ie when the MacBook restarted for set-up I did not understand that I should let it proceed until it asked me to choose a country or region. Now I have a black screen with only a picture of a folder with a ? in it, intermittently flashing at me when I turn the MacBook on.

What do I do now?