We need more information - computer specification, Windows version, program full name and version, DVD burner make/model - see this topic for more details of what is required.
In general if the burner is not seen then:
the disc drive drivers are incorrectly or not installed
the disc drive is a read only device
trying to burn with an old version of Movie Edit Pro on Windows 10.
if the disc drive is an external (USB) drive, it has gone to sleep ie in power saving mode.
How very odd, all was as above, seen by PC but not Magix. In desperation I stuffed a DVD in it so I could take a screen snap in Explorer and 'Bingo' Magix now sees the drive, can't explain that. Will now try to burn a BD, will update if it stays good tomorrow :-)