What do I do if I see a file with a question mark at Start up??

I'm just trying to wipe my computer and factory reset it so I can sell it. So many videos. So many blogs. I'm like a 3rd grade level (maybe 2nd grade) when it comes to all things tech. I think I restarted it before reinstalling the OS (in fact...I'm positive I did).

Now I just get a file icon with a question mark. Don't know how to fix this. So frustrating!!

Anyone?? Help me out?