Well, if you started fine the first time then I'm assuming you know how to do ribbing. Answer these questions and we'll go from there. If you can post a photo that would help. I have a link in my sig for posting photos, sizes, etc. (you need two posts to post photos btw)
Are your stitches increasing?
Did you set it down in the middle of a row and maybe start going back in the wrong direction?
Did you maybe start knit the purls and purling the knits instead?