You have three choices, leave it like it is (would drive me crazy); make a correct square and applique it over the block;
or gently take (rip quietly) out the square, and and turn it around. Hand sew it in and then very gently quilt it the same way either by hand or by your sewing machine. It is a shame that it happened. I call in my son or/and my husband and have them look over the quilt before I take it to the long arm quilter (I am surprised that the long arm quilter didn't catch it.) It is pretty and personally, that whoops would drive me nuts!!!