1. A bad burn
2. gunk or smudges on the business end of the disc.
If it is because of the first scenario, then you can kiss that data good-bye. It is virtually unretrievable. If it is because of the second, then there is hope. Usually all you have to do is wipe the disc to remove any obstacles that may be tripping the laser up. I usually wipe it down with 91% alcohol.
I've lost some good data because of the first scenario so I always use the data verification feature on the my DVD/CD writing software. If it comes back negative, I toss the disc and start over. If the software doesn't have a data verification feature, I copy everything from the newly burned disc back to the hard drive to test it.