Try this,
1) First check connections. Assuming you are certain the speakers are connected to the correct Speaker Out (Main L, Main R) and sub to Sub Out (this will be one of the multichannel out RCA connectors) terminals.
2) Check settings on receiver to force downmix to Stereo
3) Play non-stereo source, like a movie DVD
If you have one speaker connected to the center or surround channel speaker outs, and then play a CD, the speaker will not get any signal, thereby appear to malfunction.
If you play mulit channel audio and still have a problem making one speaker speak, swap them to see if the other one works in place of the other one. This will help understand if the speaker is shot.