DVD Player now says "unsupported video format" on videos I previously viewed in same

I put in a DVD that I had previously watched on my iMac (running 10.7.5) and the discs refuse to play. I keep getting the message "unsupported video format was encoutered." In addition, although I can inspect the media, and the Player shows the disc title, the eject option is grayed out in the menu. I have to do it manually from the keyboard. Any thoughts on how I can troubleshoot this glitch? Thanks.