Do you have a multimeter?
If you do, I would suggest putting it in continuity mode and checking for ground faults (clip one probe to the heatsink then march around the fixture, touching the other probe to each solder joint. Do this with the LEDs off and everything unplugged). If that turns up nothing, check that each LED functions as mentioned above, or with a multimeter in diode test mode.
I like to check for ground faults first because if they are present, they can really mess up testing of individual LEDs.
Also, if you can post the specific model or part numbers for the LEDs and the drivers we can probably help you choose a replacement if something is damaged.