If you notice a dropped stitch a couple of rows down, don't panic. If you are armed with a stitch marker and a crochet hook, all will be well.
First grab a removeable stitch marker or a safety pin and feed the stitch on to it so it doesn't unravel any more and work until you reach the point in the row where the stitch should be.
Use a crochet hook to work up the column of stitches by inserting the hook into the stitch loop and pulling the bar of the yarn though the loop. Then repeat for the next row until you reach the top.
If you have to bring it up several rows it may feel quite tight towards the top, especially if it is a couple of rows since you dropped the stitch but your yarn should even out in your repaired rows when you block your work.
Keep a close eye on what direction your stitch should be, ie knit or purl. I usually turn my work over so that the knit side of the stitch is facing me when I use this method for what would be a purl stitch on the right side.
You can also use this method the fix a knit stitch that should be a purl and vice versa. Consciously drop the stitch down to the mistake and use your crochet hook to work it back up correctly.