With your crochet hook, pick up that loop hanging out from your dropped stich. Then catch the bar that goes across above the loop and pull it through the loop you have on your crocet hook. If you know how to crochet, it should be similar to making a chain stich but not quite. Continue to work your way up. If you are knitting or purling it changes from which side you go at it, but you'll have to just try it out and see what happens. I can never remember.
Also, get yourself the Stitch and Bitch book. There's a whole section on how to fix mistakes. I got one for my friend that I taught to knit in case she drops a stitch or something and I'm not there to help her. My hubby was telling me about another knitting book. The Knitting Answer Book: Solutions to Every Problem You'll Ever Face; Answers to Every Question You'll Ever Ask
I haven't check it out myself, so I can't vouch for it. But it sounds like a good tool to have. Good luck!