Reddit reviews How to Diagnose and Fix Everything Electronic, Second Edition

You're really asking an electronics questions, not a camera one. If there's an electronics repair subreddit this question would be better suited there. Old electronics just sometimes conk out, but they're usually able to be repaired, and not turning on at all is often a relatively easy problem to fix, especially if it still worked a few years ago.

But you're going to need at least some familiarity with electronics. I found this book really helpful (this is an Amazon link but I got it from my local library).