iMac won't mount CDs/DVDs

[GSMiller]

My white, C2D iMac is giving me a bit of trouble. Whenever I put a CD in the SuperDrive, it starts spinning and the drive makes that infamous "grunting" noise and then everything just stops. The CD will start spinning again, the drive will "grunt" and then it all stops again. It does this like three times before it finally ejects the disc. I've tried three different discs and they all do the same thing. The last time I used the drive, it was to burn a DVD and it worked fine. The only thing I've tried doing so far to fix the problem is restarting the computer, as I do not know what else to do. Any help is appreciated.