Was the yo done on a knit row? So what do you have now? Have you ripped out and have a row of knit loops and the yarn over loop/strand just hanging in space? When you run the needle back through the stitch loops and then come to the yo strand run your needle under it so that the strand goes back over the needle like it was originally.
To learn how--
Make a tiny sample over 5 stitches. Work a couple of rows of St st and then do a yo in the middle. Stop at the end of the row after you make the yo. Notice how the yarn over sits on the needle. Now take the needle out of the stitch loops, then run it back through so that the yo is back like it was. If you need to, make two samples one with the yarn over done correctly, then another you can take off the needles and then get it to look like the other one. Practice until you understand it well, then do that on your project.
If it is is not St st do a sample like the case you have.