Learn to Knit - Dropped Stitches
A dropped stitch need not be devastating—it's an easy problem to fix if you catch it within a few rows of the drop. If you don't see it until several inches have been worked, there will not be enough yarn around the dropped stitches to fix the problem, and the best solution is to unravel the knitting back to the dropped stitch and reknit these rows.
The easiest method to pick up a dropped stitch is to use a crochet hook. Work with the knit side facing you (turn the work over if you need to because of pattern stitches).
Insert the hook into the free stitch from the front. With the hook pointing up, catch the first strand of the ladder from above it (see illustration) and pull it through the stitch on the hook.
Continue in this manner until you've worked up through the rows and then replace the stitch on the left-hand needle, being careful not to twist it. If you've found more than one dropped stitch, secure the others with a safety pin until you are ready to pick them up.