Usually, something like this is caused by a bgl file in a scenery folder. If you've recently added some, you might need to move it to another folder and see if the sim will load. If it does, then you need to look for the offending bgl which is a painintheass. I do it by a process of elimination.
If this started without a scenery addition, look for some other kind of recent addition. Other than that, I don't have a clue. Sometimes computers can drive you crazy. You might have to delete fsgw3 and start over unless somebody has an idea.