If a stitch drops off the needle, the first thing to do is to prevent it from running further by securing it with a safety pin. It is usually quite easy to pick up a dropped stitch, even if it has slipped through to the row below. Simply pick up the stitch and the strand above it on the right hand needle. Insert the left hand needle through the stitch and pull the strand through the stitch using the right hand needle to form the stitch once more in its correct place.
Hold the knit side of the work toward you. Count the horizontal strands between the two needles to determine how many rows the stitch has slipped. It's important to begin with the very first strand closest to the dropped stitch. With the loose horizontal strands behind the loop of the dropped stitch, insert a crochet hook into the loop from front to back. Catch the first horizontal strand and pull it through the stitch. Repeat the step with each horizontal strand until the dropped stitch is back at the current row.