Boot from CD failing..

Soo, basically I booted from CD earlier and am trying to do the same again.

Problem is, when I change the boot priority and reboot I just get a black screen with a flashing dash in the top left hand corner. Normally, it ssays boot ready, press any key to boot or something. So I tried using my over drive - which is a bluray one. With this, I told it to boot from the drive and it says "CD Rom: No Medium" - even after a disc is in etc (same disc I used earlier).

Any ideas why it isn't working properly - same drives, same disc, same pc etc. Thanks

Oh and btw, when I plug my Windows hdd back in, I can still boot in to that perfectly.