Why won't my lock button work?

My phone took a little dip a while back, and after about a month, a number of issues have arisen. Initially, it appeared the lock button was not working, but this problem went away. After the lock button began working properly, the screen began intermittently not working -- it would work temporarily after being unlocked, but after a while the screen would stop responding to touch and gesture. Locking and unlocking the phone would resolve this temporarily, but the problem inevitably returned. After a few weeks of this, the lock button again stopped working, this time permanently. I have replaced the screen and digitizer, but am unsure what should be replaced for the lock button issue , can anyone help me with this?