Back when I knew much less about FS9's guts, I must have mixed up sky and cloud textures once. Suddenly my sky was huge slabs of electric bright colors, not a crash but I thought well there I've ruined things and will have to re-install. Like was said, check recent scenery or texture installs especially an auto install. They put files where needed and can be hard to un-do. Depending on what you have installed, you can grab the default sky textures and re-install just those and see what happens. Last resort you can use system restore to sometime just before the problem started. Other programs/settings are affected so check to see what might need adjusting after the restore.