bulb couldve burnt out. id recommend this bulb:
Amazon.com: Philips D2S Xenon HID Headlight Bulb, Pack of 1: Automotive
replace both at the same time IF you determine the bulb burnt out. its good to have equal bulbs. also, only order from Amazon as the seller to avoid fakes, not the "Sold by Xenon Corp and Fulfilled by Amazon." crap.
or just order from here:
http://www.theretrofitsource.com/pro...roducts_id=148
our stock bulbs are koito 4200k bulbs, these philips are probably 2nd best bulb you can buy in terms. affordable at ~50bucks/bulb too!