How to sew a straight stitch for mending a pillowcase seam by hand?
Thread the needle using a wire threader, to save many minutes. Once the thread is through, double it back for strength, tie a knot at the end, leaving the thread twice as long as the pillow case. That way you won't have to stop in the middle and thread a new needle. Turn the pillow case inside out, and begin at the edge closest to you. Pierce the fabric, sending the needle away from you, and bring it back out 1/8th of an inch further, in and out, each time. Repeat until the entire edge has been re-seamed. Tie a knot at the end of the pillowcase so it won't unravel in your sleep. Turn the pillow case right side out, and voila your seam will be repaired.
Gather the pillow case edge by edge and fold it over so the rip doesn't show, or use a dark colored dye to conceal it. If the pillow case is now too small, you can shorten the pillow a bit so it will still fit.