make sure that you have the sound output set properly in the Sound preferences pane.
While the pane is open, plug in some headphones, the listing should indicate that headphones are in use. Check for sound there.
Then unplug the headphones, the system should detect that the headphones have been unplugged and indicate that the internal speakers are the output device.
If all this is working, then you probalby had a hardware failure. Call Apple and open a service case.
- gws