Hi @siwapakkap29459
It could be a loose cable connection between the motherboard and the LCD screen, a faulty LCD screen or a faulty backlight power supply circuit on the motherboard.
Here's the ifixit [[Topic:Canon PowerShot G12]] repair guide, that may help.
Use the ''LCD screen'' guide to check if the cable connection is secure (at least on the LCD screen anyway).
If it is OK and if you decide to try replacing the LCD first before opening the camera further to check the motherboard end, search online for [https://www.google.com/search?client=firefox-b-d&q=powershot+g12+lcd+screen|Powershot G12 lcd screen] to find suppliers that suit you best.