go into test mode, press the middle black button in. then press Advance. release the middle black button. keep pressing Advance until you reach the input test. press any of the buttons or move the joysticks, do any of them register? if only half of them respond, go around to the back of the game now -- wiggle the small ribbon cable around. do you hear the game making bloop sounds like it's registering inputs?
if so, the ribbon cable is bad. it requires soldering to replace. I can replace it for you. PM me if you need further help.
good luck