Stick your ear up against the drivers on the bad speaker and see if you can hear if all the drivers are working. If only the tweeter isn't working (and you're willing to break out the screwdriver), you can unscrew it and check the connections. Sometimes they jiggle loose during shipping. If both the tweeter and the midrange aren't working, check the straps between the binding posts on that speaker. One may have come loose, which would result in signal only being sent to the woofers on that speaker (assuming you've got the cables hooked up to the bottom posts).
