It's not possible that anything you reported happened "out of the blue", since the program surely doesn't modify itself, we haven't updated it in a while (except a very small Addon Manager update just to support the latest P3D hotfix, which doesn't install automatically)
And, whenever you report a problem with GSX, please be totally accurate, because "GSX saying something had happened" is not very helpful to understand what really happened.
Whatever happened, happened to some configuration changes in your system and, assuming you ARE using ALL the CURRENT version of the installers to reinstall, the most likely cause for such errors, seems to be issues with your FSX registry keys: if they gone corrupt/missing/changed/pointing to the wrong folder, you can expect EVERY kind of issues, since everything, from the installer, to the program itself, relies on such keys to know where to look for the program, the scenery library, the running mode (FSX, FSX:SE, P3D, etc.).