Despite the hole in your work, dropping a stitch doesn’t have to be a drama.
How to spot a dropped stitch
Scan your work for a something that looks like a run in a pair of stockings.
How to pick up a dropped stitch in garter stitch
Working in garter stitch means you knit every row, as a result the other side is a purl.
Knit your current row until you reach the point of the dropped stitch. Take a crochet hook and look at the rest of the row to figure out which stitch you are working with stitch. If it's shaped like a ‘V’, it’s knit stitch, so the next stitch is going to be purl.
To create a purl stitch, pull the first ring of the ladder in front of the dropped stitch using your fingers. Place your hook through the loop and pull the ladder to the back.
The next stitch will be knit. Put your needle through the loop, grab the next ladder and pull it through to the front.
Continue this until you’ve reached the top. Place your last stitch on the left needle and keep knitting.
Like most things, learning a new a skill is easier with someone showing you.