If your external or internal DVD Drive is not showing up in Windows Explorer, it could be a few things that caused this to happen so check for the following.
Is your computer under a desk? Look at the computer and make sure the internal DVD Drive is still there. If you are in an office environment where an IT Department supports your computer, it is possible they may have taken it with them. Why? Numerous reasons but one may be that you were caught downloading software with the company’s Internet connection and then burning them at work with the computer for use at home because that goes against the rules of the company along with putting them in danger of infringement if the software is illegal.
Data and Power Cable
This will require some strength (and sometimes tools and keys) depending on the type of computer. You will need to open the case of the computer and see if the (IDE or SATA) data cable is connected to the DVD Drive and motherboard and the power cable leading to the Power Supply. These are not easy to come loose (unless broken) so if your hard drive was being scanned for threats in the IT Department, when the drive was returned and installed, one or both cable could have been left off thus causing the issue.