|Try cleaning the laser lens on the optical drive with a laser lens cleaning CD. If you don't have one, most places that sell CDs or DVDs have them - even some "dollar" stores have them for a buck or two.|
For a laptop or netbook, it's usually easy to clean the obvious laser lens with a tissue or a soft clean cloth when the optical drive's tray is ejected and has no disk on it.
If you have two or more optical drives, try the disk(s) in another optical drive.
However, many bioses will only boot from a bootable disk in one optical drive. If a bootable disk is not recognized while booting, go into the bios Setup and find the list of optical drives - it's often near the Boot Order or similar settings - the drive you want to boot a bootable disk from must be first in the list, Save bios settings.