I think that happens to all of us, we all have a limit to what we can remember in detail, even things we have been very deeply involved with.
My weather station's web site died a couple of days ago, which I set up years ago. It quite complex, the outdoor sensors send data to a base station wirelessly, then that relays to Rasberry Pie. All working fine and weather data showing on the base stations screen, but the Pie is just flashing it's data light. What to do next? When I first set it up, I would have known instantly what was wrong and how to fix it, but now I'm completely stumped and will have to start from scratch.