Can you press and hold down the power button to turn off the machine then wait 5 seconds andthen press and hold the mouse button down and power up your machine, Keep the mouse held till the CD hopefully ejects and let us know if that does not work
What model is your older mac?
If it is a Power Mac g4 then can you try shutting down the computer then carefully pull down the door covering the front of the drive tray and locate the small emergency eject hole.Carefully insert the end of a large, straightened paper clip into the hole, and push firmly until the tray hopefully opens, Next pull out the tray then remove the disc and push the tray in gently to close it.
Let me know what happens
Well it sounds like something has gotten messed up.
Do you have your original Tiger disks?
If so can you try booting from them using the C key and see if it will open to the Installer so you can run disk first aid.
Let me know if you have the disks
For some reason it is not seeing your hard drive, If you nowtry and boot your mac with the option key held down do you get the ? and mac
Can you tell me if you tried to upgrade your G4 or were you just booting the DVD to start?
Can you shut down your machien then wait 5 seconds and power it back up.
As soon as you let go of the power button can you try holding down the command+option+P+R keys till you hear the startup chime sound twice then let go and see if it will boot
Let me know
Let me know what happens when you run the hardware test disk
Let me know what the other disks say when you get a chance