Auto Home Z-Axis problem

I got the printer to function properly with no errors, but during calibration, when the motors run along the Z-axis, it doesn't stop at the top, and the printer tries to squash the extruder against the top frame rail. I wanted to know if I was doing something wrong here. I apologize if it is a "dumb" question, this is my first 3D printer, so it is especially daunting because of the assembly. Thanks. (I'm not sure if it is the Z axis, but it is during the up motion.)