If it's not the optical drive itself, try a new SATA cable and power-cable ... on a different SATA-port header.
Sometimes, you can check optical drive with ePSA (outside of Windows).
If you think it might be Windows, sometimes deleting the drive from Device Manager and letting it re-detect fixes that glitch.
Most newer drives will not see the DVD unless the SATA OPERATION is set to AHCI not ATA.
The other issue that affects this is the drive region code.
Once you install XP from CD and change the region code to 1 for usa the drive works normally again. In other words it will not see your Vista or 7 or 8 or 10 dvd as bootable until the region is set.
If you don't have the CD to install you could do Linux CD to set the region code for the DVD.
Download LupuPlus 5.2.8.005-1 -> http://diddywahdiddy.net/Puppy500/lupuplus-528.005-1.iso
how to setup dell optiplex 755 BIOS SATA OPERATION to AHCI?
You press F2 after turning it on and then go to the section in the bios under DRIVES then SATA Operation.
Diskette Drive (Internal default)
This option enables or disables the floppy drive. The options are Off, Internal, USB, and Read Only.
NOTE: If USB is selected, ensure that USB Controller setup option under Onboard Devices is set to On.
SATA 0 through SATA n
Identifies and enables and disables the drives attached to the SATA connectors on the system board and lists the capacities for the hard drives.
NOTE: These options appear as SATA 0 through SATA 3 for the mini tower, SATA 0 through SATA 2 for the desktop, SATA 0 and SATA1 for the small form factor and ultra small form factor computers.
Identifies and enables and disables the drives attached to the eSATA connector on the system board and lists the capacities for the hard drives.
NOTE: This option is not available on the ultra small form factor computer.
(RAID Autodetect/AHCI default for mini- tower and desktop)
(AHCI default for small form factor and ultra small form factor)
Options for Mini Tower and Desktop:
l RAID Autodetect/AHCI (RAID if signed drives, otherwise AHCI)
l RAID Autodetect/ATA (RAID if signed drives, otherwise ATA)
l RAID On (SATA is configured for RAID on every boot)
NOTE: When in Autodetect mode, the computer configures the drive as RAID if a RAID signature is detected on the drive. Otherwise the drive will be configured as AHCI or ATA.
Options for Small Form Factor and Ultra Small Form Factor:
This setting determines whether integrated drive errors are reported or not during computer start-up.