I would say first to check that you have the correct firmware installed that allows the DVD player to read what the DVD/CD's have on them. \
For instance, if a DVD movie is inserted into a DVD player, you have to have the right movie viewing program to watch the movie, if you don't have the right program, or "player" or "firmware", then the DVD simply seems to be unreadable. You may be able to see the data on the DVD in your Explorer, but cannot do anything with it.
Then your drive may be bad, or is simply not compatible with the way the data was written to the drive. Unfortunately, this can be a problem sometimes as there is no clear standard as to the way data may be written to a DVD or CD, there are many variables, and many programs that make DVDS and CDS, and some players can read certain or all discs, and some cannot. That is just the way it is, and there is not a thing you can do about it other than trying another drive, or if this is a burned or re-written DVD, try burning the original in a different format or even on a different brand or type of media. A lot of it has to do with "+ and -" and whether the disk is a R or RW, and even past that, the speed it might have been written at, and it is not an uncommon problem.