The HD disappear from the startup disk menu

I'll sell my iMac, so I decided to erase and restore to it's original OS. I restart from a alt+cmnd+shift+R and when the disk utility open I saw the disks names in red (I have a Fusion Drive iMac) so I hit the repair disk button. At the middle of the process bar the process stops for almost an hour so I tried to stop the process hitting the shut down menu, but the beach ball shows up. I push the start button and now the computer shows a folder with a question mark in it, if I try to reinstall the OS the disk isn't on the startup disk menu, but on the disk utility app it shows only one disk and after the repair process says everything is OK.

Now I don't know what to do.