What should I do if 20M generator screen is whited out over half of its surface?

You can simply change an LED if you can do soldering or a display board if soldering by yourself is impossible.

Please check the video for replacing LED display:
https://www.youtube.com/watch?v=y-FHPZfF60c

Here is the video of changing Led:
https://www.youtube.com/watch?v=68gRKDJGesI