Those little endstops have a seriously important job and it is really noisy when they fail!
The endstop is the little leaver or button that are pressed when the machine moves the extruder or bed to the extreem in the given direction. So in the X axis it is when the extruder moves to the far right. The Y axis when the gantry moves all the way to the back; and in the Z axis when the build plate rises to the top.
They do, from time to time, get knocked or just work there way loose and pivot on the 1 screw that holds them in place so that they are no longer pressed when they should be.
But it is also really really easy to diagnose and fix - check out the Tech Support handout attached