Boot Disk Error

Hello,
i'm trying to reformat my computer but when i put the disk in it says boot disk error insert disk and press enter. i'm confused it's worked in the past. plz help. I have also set the priority to cdrom so that it boots 1st but even with the disk inserted when i restart it just starts up normal and when i put cdrom for all 4 boot options it says boot disk error insert disk and press enter but i have the disk and it's not working i was thinking maybe my cdrom isn't reading the disk. thanks