Smashed chain link fence?
You are not going to be able to restretch it if it is as smashed as you say. What you will need to do is cut out the bad part and insert a new length of fencing. You attach the new piece (splice) by taking one piece of wire from the fence and winding it through the existing fence and the new fence. This wire stitches the two pieces of fencing together.

Once you get the smashed portion replaced with good fencing you can restretch the whole length and attach it to the posts as normal.

pic from fence-material.com