Yes, as cannabishead suggests, make sure all of your cables are fully connected. Also go through the receiver settings and make sure it's still configured properly for 5.1 surround sound. Some listening modes, such as 'Music', on certain receivers will only push audio through certain channels.
Some other things to think about are 1) this is a cheaper unit and it's quite possible the amplifier that drives the center and rear speakers has failed; 2) if you've had any power outages/surges/brown-outs in your neighborhood they can easily kill electronic components.
I'm not 100%, but it looks like your sub is self-powered, which would explain why it's still working.