open her up and make sure everything is solidly connected... after that you can trouble shoot parts.. if they use the same drivers for each half of board, try swapping the driver wiring. if they are not the same don't do this as you might damage some diodes if too much and might not light up properly if not enough amps/volts provided.
if the part works for the other half, you know it's not the driver. now you gotta figure out if there's a short in the boad or not, i guess? replacing a driver or reconnecting a loos wire is easy.. a pcb board with diodes soldered to it is another thing.
before you do that, if in warranty period, contact mars.. if out of warranty, go ahead and open it up and fix it yourself.
if you got good use out of this, consider a newer light with samsung diodes. for less than a dollar/watt you can get some high end mars FC-XXXX lights. 3000 / 4800 / 6500 are all well-priced right now... the efficacy on those is 2-3 steps above the other models mars offers (half step above the bridgelux diode fce-xxxx models, which should be a bit less expensive and vialble option too)
for same watts, it'll produce more PAR. you may need fewer watts than before for the same area of coverage, which only saves you money every month for as long as you use it.