Sounds like you need to open this up to see what the issue it so others can help more.
If I had to guess I would say the pad was slipping but now it sounds like the clamper is broken. But then again these are all shots in the dark until you open it up.
take the bonnet off and check the spindle motor. there is a black three legged plastic on top of the motor to center the disc. this holds down a spring... if one leg breaks....