Why won't my game work on my Xbox 360?

My copy of Dragon Age:Origins has been working for a long time butI tried playing it today and it wasn't working. The game screen would come up but then it'd say "disk unreadable" and tell me to clean the disk. I checked to see if it was just DA and it is, what's wrong with the disc? I don't know why it's suddenly unreadable when it's had no problems for a long time, since I bought it :/