Command+R not working, Command+R+Alt/Option not working, Shift+Command+R+Alt not working

This is driving me nuts.

I've reset everything on my iMac to give it away. However, once I booted it up, it showed a flashing question mark folder.

So I looked it up and all help said was turn it on and hold down Command+R, if that doesn't work then use Command+R+Alt, if that doesn't work try Shift+Command+R+Alt and even when I hold down Alt on its own, no hard drive appears.

Please help.