After downloading Yosemite my Mac mini hanged and after the reboot it's stuck on a grey screen. After a while I see a question mark folder forever. I have tried many things without any success. I don't have a wired keyboard that might be an issue because it looks like it doesn't respond to any command. I have a wired Microsoft keyboard too that I tried with no success. Do I need a wired Apple keyboard ? I also tried to disconnect my hard drive so that it might try to boot on another device .... No succes again. Does the cheap compatible MAC keyboard on Amazon work ?? What else can I try? Thank you for any inputs.