I just started knitting, learning from a DVD. I learned how to cast on and start a simple, basic piece using the “english” style. When I was on my first row after casting on, one or two “loops” slipped off the needle and I couldn’t figure out how to get it back on to continue knitting, so I unravelled the whole thing. It seems so simple for it to slip off, so I’m sure it must happen all the time to knitters, and it seems there must be a way to get it back on without starting over. Any help is appreciated!!
Welcome to knitting and fixing mistakes! 
If the loop comes off and nothing unravels, you can just slip it back on.
If the stitch ‘runs’–loses a few stitches and moves down your knitting, you can fix it with a crochet hook. In fact, Amy has several ways to fix mistakes in the Basic Techniques/more section above. They’re very helpful, because we ALL make mistakes.
I’ve had the same question…the crochet hook works well for re-capturing stitches several rows into your work, but what if the actual cast-on stitch falls off and unloops? I’ve had it happen when I’m a row or 2 into a project, so I end up unraveling the whole thing, like Aimee.
If the cast on comes off, I redo, also.