Make sure the power cord is securely connected.
The power cord may have come loose causing intermittent power. Make sure the power supply is firmly connected to the system and the power supply is firmly connected to the wall outlet. For more information on attaching the power cord, see Connecting to power
Check that one wireless receiver switch is set to L (left) and the other is set to R (right).
On both wireless receivers, there is a L/R switch to select which speaker is your left rear speaker and which is the right rear speaker. Be sure both receiver switches are not set to the same position. While facing your TV, the speaker on your left should be set to L and the the speaker on your right should be set to R.
Reset your product.
Much like rebooting a smartphone, your product might need to be reset on occasion to correct minor issues. For more info, see Resetting your product