You cannot undo a bleach stain. It has taken the color out of the carpet fabric The bleach may have created a stain of its own Check the fabric the carpet is made of if you can. If it is natural fabric ....cottons or wool or blend you can try rit fabric dye, diluted WAY DOWN and spot apply. Let dry and reapply to darken till you get close. Poly or nylon probably will not work as well. Be prepared to wind up with a stain that is worse and more noticeable.
Can you cut out and patch from a spare piece of carpet ???