Jennifer it might be your drive is dying but it is worth just trying to manually clean it, I had similar issues followed the procedure outlined below and to date have not had the issue recur so it is worth a try, I posted this back in Nov 2011:
I have had the same issue as lots of others with my HL-DT-ST DVDRW GSA-S10N optical drive for a MacBook Pro being very temperamental only reading the odd DVD not burning anything and not reading any pre-burned discs. Those it did not take a liking to it simply whirred and then popped them straight out. I had a look at one solution which suggests taking your drive apart and cleaning the lens (see
http://muzso.hu/2008/08/17/how-to-clean-the-lens-of-a-slot-loading-optical-drive -a-macbook-pros-superdrive)
A bit of a no-no if your a technophobe like me who ends up with more parts than he started with when re-assembling anything.
But then like daidallein found I came across a post suggesting cleaning the lens with the help of a credit card. I used a loyalty card, wrapped an old glass lens cleaner from my glasses case around it and soaked it in windowlene and inserted it carefully in the left hand side of the lens and pulled it to and fro carefully a few times.
Like magic the drive now reads perfectly, I will post how long it lasts but have no reason to suspect it won't last for the 20 months another poster has suggested. It is probably better to use alcohol as opposed to windowlene if you have any handy simply because it will evaporate dry better.
Given this does seem a genuine fix to a problem many have been complaining about I am surprised Apple have not put it up as an official solution on their troubleshooting section. It would save a lot of time for people like me who had to trawl through the forums to get to this point, and who thought it was a genuine drive problem that Apple were not addressing
Hope this helps others. (the link I used like daidallein is:
(http://eatmyrandom.blogspot.com/2010/06/macbook-pro-superdrive-wont-read-any.htm l)
PS also try cleaning the DVDs you are using. Even badly scratched ones can be retrieved with a bit to tender loving care!