Is the iMac running a new enough version of MacOS so it can do an Internet startup? That should bypass most of the internal operating system so you can decide if the problem is hardware—a bad iMac screen, or a software problem. Any way to connect a second monitor? Yes, difficult to do too much when you can't see what you are doing.
On a Mac since 1984. Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
