Car wipers not working?

My Nissan Micra 1993 1L is not working. I can hear the motor trying to move the wipers. When I unscrew the bolt to see what's causing it, the screw (or something) dropped down into the enclosure.
How do I get into the enclosure? Do I have to tear open the entire panel from inside the car or do I open it from outside?
How to do those?
Please help. Thanks.