Hold the option key as you start up to invoke the All-in-ROM Startup Manager.
It will draw an icon for each bootable Volume, then the Eject key is "live" and it can be used to open the drawer and insert a different DVD, such as an Installer/Utilities DVD if that is your wish.Mac Pro (Early 2009), Mac OS X (10.6.8), & Server, PPC, & AppleTalk Printers
I did discover the "how to retrieve the disk" part
first, gently pull the door downward and hold it down
second, take a slightly curved, paperclip that is bent to a slight curve so as to go down and into the access in the drive.
third, as the tray moves out only a half inch, pull with a little finger on the right side to open it
remove the cd/dvd
I inserted a retail X10.6 .... hold C down and boot, still nothing but a plain gray screen, no frown face, no question mark.
might be a USB conflinct or failure,
remove all connections except the power and monitor, boot up, = same deal; just a gray screen.
removed all HD, except ssd & install disk in dvd drive = gray screen / DVD spins just for a moment on start, no drives spin, only thing moving is the fans, they are all on as normal.