Mac app will no longer open
It could be that something in the .app package was damaged or corrupted over time. It will be difficult to tell. If you have a Time Machine backup you can try restoring the earliest copy of that app that you have.

You can open up Console.app and look through the logs and see if there is anything about that specific app in any of the myriad log that macOS keeps. That may at least give you more info why it is not launching.

If it is an app store app, re-download it. If it is unavailable perhaps there is a file inside the app package that indicates who the developer is.

If it wasn't an App Store app I would go looking for it online. Often you can find older copies of an app online line.