Main Menu Buttons SetAtive(true) not working

I have made a Main Menu Scene where at Start Buttons and Text are invisible. After a short time these activated and that works fine when I start game. However, when my game finished and I go back to Main Menu these Buttons and Texts remain invisible. My Main Menu Script is as under:-

I need help in this regards as I am Beginner in Coding

Use unity functions OnEnable, OnDisable

OnEnable gets called when the script becomes enabled (gameobject gets turned back on)
OnDisable gets called when the script becomes disabled(gameobject gets turned off)