When you tear part of a muscle, as it heel it is shortened. Imagine you tear a piece of paper in half, to ensure no gaps as you put it back together, the best way is to overlap a little. This is how a muscle heals. Also the scar tissue that heals the gap is non-elastic, so difficult to stretch. As a treatment I would use deep transverse frictions to help this new tissue align properly to aid in the heeling, but good stretching throughout would maintain the muscle length.

What you need know if a lot of stretching. As you pull your foot towards your bum, you need to keep your back straight and not bend forward. If you bend forward you take the stretch off. This is more important that getting your knees together initally. Then as the movement returns, gradually bring the knee into line with the opposite side. The last addition when you have got to this is to push your hips forward. Hope that all makes sense. When you are stretching, you will feel a pull in the muscle, hold the position until this feeling decreases, then add a bit more stretch onto it. Continue to this each time the feeling decreases until it feels looser. This technique will help to put length back into the muscle.

Are you doing any exercise at the moment. If not I would suggest cycling, again to loosen the muscle and make sure you stretch after as this is when you will get most benefit because the muscle will be warm.