what kind of hole is it?
is it a dropped stitch?
Is it an inadvertant YO?
not sure? count your stitches.. one short, a dropped stitch
One to many? a YO.
How to fix.
If it is on the sole, work VERY CAREFULLY
cut a length of yarn--about 18 inches, and thread onto a tapestry needle
carefully weave the tail end of the yarn into the fabric of the sock, (rather than knot it)
then (if a dropped stitch) stick the tapestry needle into the loop from behind. (the sock should be half inside out (so you can work in it, and half right side out (so you can see what you are doing!)
so the tapestry needle is inside the sock, it passes through the loop of the dropped stitch and is on outside of sock.
then holding the needle horizontal (the way a knitting needle is horizontal to work --) go into the stitch above, and back out 2 threads later. (this is a single down up motion, like a running stitch)
then go back into the loop of the dropped stitch.
(HOLE is closed)
(you've sort of made a duplicate stitch--)
(you can weave in the tail or make some more duplicate stitches, (to the left, to the bottom, to the right, ) Making a box around the hole (8 or 9 stitches in all! (see the V's, --the hole will not perfectly line up with row above..
weave in the tail, and cut off any remaining yarn
if you made a yarn over, the process is the same, only you start at the stitch below the hole.. (and then work your way round the same way.. making duplicate stitches all round.
finally check/correct stitch count and make an increase or decrease someplace where it won't be obvious.