I had a Singer Golden Touch & Sew made back about 1969. After years of faithful sewing, it quit doing the zigzag sewing but did a very balanced regular stitch. In my case, a plastic wheel type something inside had the teeth worn off it & couldn't engage the zigzag stitch. Several hundred dollars later, it is like a new machine. Perhaps something similar has happened to yours, or maybe it's something as simple as a button or lever that needs to be pushed or moved. I wish I knew your machine so I could really offer some helpful advice, but I don't.
Good LUck!