When you changed 'D' to'Z' Windows must have assigned a different drive letter to the DVD drive.
Open Control Panel, and select 'Folder Options'. Make sure 'Hide empty drives in the Computer Folder' is unchecked. Then close Control Panel, and open 'Windows Explorer'. The DVD drive should be listed there with a drive letter assigned by Windows. Make sure that all your programs reference this drive letter for playing DVDs. Or just double click on the drive and the program you want.