After you look at the article in Harm's link, check your video driver. It's likely that an OS update has killed it. Get the latest from the video card mfgr's Web site and install it. If that does not help, see the "Questions" section on Harm's article and please furnish the details requested there. The better a job you do with those questions (after trying the troublehsooting tips, BTW), the sooner somene can help you meet that deadline.
Is there any chance this is a CS3 or earlier project you are opening in CS4? I had the same problem when I did this. The way to fix it is to remove the filters that were applied in an earlier version that does not exist in CS4. Once all are removed it will start playing.