Why would my 3 year old IMAC stop playing DVDs and what can I do?

I wanted to watch GWTW so I put the DVD in the Imac and guess what came up? A list of the soundtrack songs and #1 began to play. It sounded great but I wanted to watch the movie. So I put another DVD in and got a blank screen.
Anyone know the easiest, fastest way to fix this ? I do not want to take the computer into the “shop.” Obviously!