Have you been able to play movies in the past with this drive?
What operating system are you using? If you are using XP, you should know that XP does not include a native dvd decoder, and you must use third party programs, like PowerDVD, WinDVD or VLC to playback dvd movies.
Does the disk show up in My Computer? Normally the name of the disk will appear on the drive icon there.
Have you tried one of the free media players like VLC? It is self contained and does not rely on outside codecs. Another one is Media Player Classic HomeCinema.
When you say yellow flagged, are you saying that the drive now has a yellow exclamation mark next to it in Device Manager? If so, you might try uninstalling the drive in Device Manager, reboot and let the operating system find the drive again.
Is this a SATA interface drive or IDE? If it is IDE and your operating system hard drive is not on the same IDE channel, you can try deleting the IDE channel that the drive is on, rebooting and letting the system find the "new" hardware.