When looking for attached hard drives, USB flash drives or CD and DVD drives using Windows Explorer, you may notice that some drives are not seen in Windows Explorer or the drive may disappear after a short time or when Windows resumes from Sleep or Hibernation.
This issue can be caused by any of the following issues:
Your drive does not have a drive letter assigned to it in Disk Management
The drive may be disabled or offline
Your USB driver may be corrupted
Corrupted registry keys may be causing issues with connecting to your CD or DVD drive
To resolve the issue, run the Hardware and Devices Troubleshooter and see if it helps you. Also ensure that the drive has a drive letter assigned in Disk Management.
If it doesn't help you, you can see more troubleshooting steps in KB330140.