About four years ago I made the Queen Silvia shawl from Nancy Bush's book Knitted Lace of Estonia. I loved working on it, and it turned out beautifully, but I hadn't washed it since then. I recently soaked it in some woolwash (by hand, in my tub) and then re-blocked it.
During the blocking process, I discovered a hole. Because it was in garter stitch, the hole looked bigger than it actually was. Only one row had unravelled so far, across three stitches. I started by picking up all the loose stitches on stitch markers.
I did some research and found this website to be the most helpful: www.knitty.com. I used the weaving method. I had some leftover yarn from when I had originally made the shawl, so I cut a few inches of that and threaded it on a blunt tapestry needle. After spending a few minutes staring really hard at the rows above and below, I eventually figured out how to thread the yarn through the stitches so that it looked like knitting.
It helped to have knowledge of the kitchener stitch (www.knitty.com). Although the kitchener stitch duplicates the look of stockinette stitch, and I was duplicating garter stitch, I still felt like it helped to have that background knowledge.
My mending really does look just like garter stitch. The only thing that makes the area stand out a little is the double thickness of yarn where the old and new were merged together.
When I originally made the shawl, I was relatively new to blocking. This time, I stretched it a little more to make the shawl more open-looking and to accentuate the tips.