I used to fix antique leather furniture.
If the thread has gone, not the leather, get a curved needle and thick waxed thread and re-stitch it using the original holes.
If the leather has ripped you need to remove the panels if you can, if not push/slide a large piece of calico under the leather on each side and glue with copydex. Then stitch back together using the original holes. The further the calico covers the stronger the mend should be.
If you need any more info just ask

I've just had mine done. It was insured for five years so i got it done under the insurance. He made a fantastic job of it! One of the arms, along the seam was coming unstitched and he fixed that in seconds. The other one had a great big rip down the side of the sofa and he fixed that aswell! I was really impressed.

ours has gone along one of the seams and is getting worse by the day. we really cant afford to bin it and therefore the entire three piece suite at the moment!