For everyone having this problem, here's the update. There is an app which is malfunctioning and it creates a screen overlay barring other apps to use the send area of screen. It not only disables the send buttons but also disables the touch screen response from that area. The only way to make it work again is by restating the phone. But that works only for a few hours and the issue reappears. During the time you face the issue, notice that you cannot set permissions for any app and you get the screen overlay error which lead me to believe this.
I'mfurther investigating it and will update as and when I resolve this. This is an issue caused by a rogue app and there is nothing wrong with hardware.