If it was playing fine and just stopped working while you were playing it, it's possible the speaker has failed, or a wire has come off. It is likely something you cannot fix yourself, and should find a local music store, like Guitar Center, who can repair it for you. It's generally not a very expensive repair at all.
If the sound stopped working from one side after you were playing with some controls or editing some voices, you may have set the panning all the way to the right, which would cause no sound to come out of the left speaker. In that case, you would simply need to set the panning to the center again while in editing mode.
If you are not too familiar with the editing features on the PRS 330, and not sure how to set the panning to the center again, you could always simply reset the unit to it's factory settings once again, while will return all the voices, effects, panning, etc. back to like the unit was when it was first taken out of the box and turned on. Here's the process to reset the Yamaha PRS 330 back to it's original settings -
Restoring the default (factory-programmed) settings
While holding the C7 key (right-most key on the keyboard), press the power switch to turn the power on.
This operation erases backup data in this instrument (including backup settings in the Function mode) and
restores the factory default settings. The User Songs and the external Songs stored to internal memory will be
None of that works and you do need to have someone look at the unit, here is a link to the Yamaha Authorized Service Locator to find the nearest authorized service center near you -