handbrake not reading dvd from optical drive

i am trying to rip a couple of dvds to my mac book using handbrake. everything was going fine for a while. then on one cd, it would start the scanning but would freeze. tried mac the ripper and had the same thing happen. ejected the disc, tried with a different one, with the same results. put in yet another one and it was not recognized by the computer at all. now the dvd's are being recognized, but still freezing in the scanning stage.