The Power Button clicks, but it is not working

Greetings,

I have a problem with my Iphone 4s, black 16Gb from Sprint:

Ever since I replaced the digitizer/screen the sleep/wake button (on top of the device) is not working. I can feel and hear the click of the button press and the proximity sensor works since it goes black when I make a call and my face is close to the device, but the button itself does not function. The only way that I can turn on my phone is by charging it and hitting the home button.

I would like to know what I have to fix, before I start opening the device and buying parts.

A video with a fix to my particular problem would be great... thank you in advance.