Stereo Power/Volume button popped out

Strange occurrence the other day that I have not been able to remedy.

The volume/power button that can pop in & out on the dashboard hadn't been used of late but when I went to depress it, the button popped out. It is spring loaded and the knob itself has an end that is cut into quarters - persumably to fit into the female end within the JBL stereo unit itself. But can't figure out how to get it to stay in place as it continues to pop back out. Volume & power functions still work but button/knob won't stay in