When you boot your system, you should have the option to press a function key to manually select your boot device. If so, when you select your optical drive, will it proceed?
It is also possible that you don't have your data and/or power cable connected securely. Check both ends of the cable (you may even need to change to another power or data (SATA or IDE) cable. If this doesn't help, you likely have a defective optical drive.
Optical drives do wear out and go bad. Fortunately, they are very inexpensive and are widely available today. Check the cables/connections first.