I have a couple of ideas.
1. If it has had a power loss recently the receiver could have reset to factory presets. Go into the setup menu and make sure the Center, Surrounds, & Sub are all on.
2. If you moved the receiver lately possibly the speaker wires got plugged into the wrong hole. BIL had the problem for 5 months one time. Bad part is his system was so bad he actually couldn't tell the difference, until I pointed it out to him.
Hope one of these works.