second guess.... do you have speakers/monitors/headphones hooked up to it? A common misconception for new users is that an MPC will work straight out of the box. In which technically, it does... when you got something to listen to it through.
this happened to me one time after transfering samples from the computer to the mpc via the usb connection - after the transfer was finished the mpc would not play any sound, not the new samples, not the old ones, not the factory sounds. no sounds in sample preview, no sound if i loaded up a whole project file. the sound finally came back after restarting the unit.
also check the levels of your samples in program mode - if you use samples or programs created by other people sometimes the levels are very low.