04 Nissan Maxima undr attack~~`?

hey guyz.. i have a 04 nissan maxima wit abt 57000 miles on it.. te prb of this car is tat whn i start the heating of this car... the penger side works perfectly.. there is no problm of heat coming outside from those vent but the problem is from the drivers side.. cuz whn i open up the driver side vent.. cold air comes in.. it'z like the whole side of the side A/C is broken.. any suggestionz .. please do let me know.. i want to get tat thing fixed.. asap... let me know wat you got...