Step one is to make sure that the left speaker isn't the problem. Connect it to the right channel output to make sure that it's working. Make sure that the speaker wire is properly connected at the receiver and speaker too.
Step two is to make sure that your source isn't the problem. Does the problem happen with more than one source. Use the radio since that is internal to the receiver.
After you have made sure that the receiver is the problem there isn't a simple user repair. You could remove the cover and check for blown fuses but there may not be any. If you find one replace it with the same value and type of fuse. If it blows don't try again because that can cause more damage. At this point it will require a technician.