You will need to troubleshoot it a bit. It could be the audio source, the wiring, or the speakers themselves. You could swap the connections at the source, like disconnect a working connection and attach it to the problem speaker to see if it works -- if it does then the source is the problem. If the speakers still do not work, then safely climb a ladder and pull the speakers down, they will usually come down enough to find the connection and attach an alternate input source to the speaker to see if it will work, if it does, the wiring is the issue, if not it is the speaker.
I think that you will understand what I am saying if you just think about the potential failure points and think about how to test each.