Blank White Screen

My brother’s business uses WP for his site and it seems it was running fine for a long time and suddenly he went to check it and it is just a blank white screen.

I read a little bit about this, but he had not (at least manually) changed anything himself in the past year at least. We are not sure how long it has been a blank screen but it is definitely not from him manually installing a theme or plugin or anything like that.

I am wondering how I would go about fixing this?

Any info you need let me know I would love some help