If you've set boot priority in BIOS and you can't get the machine to boot from an optical drive with an OS disk, do a complete reset of the bios (google your motherboard and manual / look up instructions therein). Some machines have an external CMOS reset button (back of machine) / check MB manual. With power to the machine, press that button in 5 seconds, release, power on machine. Sometimes a CMOS reset can get you back into business.
If this was a new build, swap out components / try a different optical drive, try a different SATA port. Check that the hard drive or SSD that has the OS if you already installed is in slot 0 on MB / check manual. Every SATA slot should flow in order; 0 for SSD or main OS drive / then 1 for second drive / then 3 and 4 etc for optical (DVD) and any other stat devices. It's important on some boards that the drive with the OS is in slot 0.
If this machine was working previously and has an OS installed (ie windows 7 or 8), start removing things (thinning down possible conflicts). Then add back in 1 by 1. If you just added an internal hard drive, leave it in place, but pull the SATA plug. Fire it up. You can boot the computer with the case open. Don't add or remove drives or plug or unplug inside the machine without unplugging the machine from wall-power and also grounding yourself on the case. Don't do computer work on carpet or metal surfaces.
Work backwords. Check the SATA ports. Go full BIOS reset. Be careful about booting from OS disk as it's easy to follow prompts wherein you end up reinstalling OS and losing your data. good luck
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