White screen on iMac
Getting plain white screen on iMac (old model) WITHOUT any folder icon
Windows, Windows 10
Welcome to Apple Support Communities.
First, take a look at If your Mac starts up to a blank screen - Apple Support and follow the steps.
If it still doesn't work, it sounds like a hardware issue. And since Apple or an AASP cannot repair it anymore since it's obsolete, it might be time for a new Mac.
-Jack