Unless it's just a few rows you really do have to start over. Beyond a couple rows there just won't be room for it anymore.
That said.. fixing garter stitch isn't hard, but it's more complicated than fixing stockinette. When you have to fix garter you have to work on both sides of the dropped stitch. Once side will create the "purl bump" and the other creates the knit. You ARE just knitting, but that's the way it works. Here's a that might make it easier for you than trying to explain it.
I deleted your other post because it was a duplicate of this one. This is the correct place to ask questions.