Use this repair when the seam thread has broken and both fabric edges are intact. Backstitch along the existing seam line and overlap the good stitching at both ends. If either fabric edge has torn away, a line of stitches through the old holes will have little to hold; repair or reinforce the cloth first.
Turn the garment over and inspect the gap
Look at the inside of the trousers, skirt or woven shirt. Pull the two seam allowances apart just enough to see the cut edges. Compare the gap with a sound stretch of the seam. If it is a plain seam with both allowances visible, bring the garment faces together and match any intersections or checks. If the gap runs into a pocket, zip or lining, find which layers actually belonged to the seam before sewing.
Restore the old seam line
Use matching thread and a hand-sewing needle suited to the cloth. Remove only the loose ends that get in the way. Anchor the thread in the allowance before the break. Work a backstitch by bringing the needle up ahead of the last stitch, then taking it back to meet the previous stitch on the front of the seam. Use short, even stitches through the two original layers. Continue slightly into sound stitching beyond the split and secure the thread on the inside.
Finish the edge only if it needs it
If the original overlocked or bound edge is still sound, leave it in place. Where an otherwise sound raw edge has started to fray, tidy and secure loose fibres without cutting into the allowance. A frayed edge almost at the seam line needs support; stitching farther in can narrow the garment and strain the repair.
Check at the point of use
Open the seam flat with your hands and look for gaps under a gentle pull. Turn the garment right side out and check that the seam runs smoothly through the repair. On a stretchy knit seam, practise the fabric-appropriate stitch first; a firm woven backstitch can restrict a seam that needs to stretch.