GE Fanuc Logicmaster software for Series One, Series Three, Series Five and Series Six used .LAD as the file extension for the ladder logic files. As others have mentioned, they are not the only brand to use that extension, but if that's the only file you have, it's enough for those GE Fanuc models.
Post it here and we can try to open it and see what works. You will have to rename it with a different extension. This forum won't accept attachments with a .LAD extension.