Using a tow rope to pull another vehicle can damage or even completely cut the line in two. Here is a simple method to repair the line so that it can be used again.

[edit] Steps

Inspect the rope for damage or wear.

Damaged Tow Rope
Cut away the damaged area by cutting the rope in two.
Cut away the frayed end of the usable rope.
Scrunch the rope on the damaged piece.
Remove end from damaged rope to remove hook.

Pull out the end of the rope from inside this damaged piece.
Remove the hook.
Tape end of usable rope

Use duct tape to taper the end of the line that is usable.
Loop end through hook

Loop tapered end through the eye of the hook.
Scrunch end of rope to make an opening.

Scrunch the end of the rope leaving about six inches of rope.
Use thumb to make opening.

Use your thumb to separate the braids to make an opening in the rope.
Insert end inside of rope through opening.

Insert the tapered end of rope inside the opening.
Pull to tighten rope inside itself.

Once the rope is completely inside itself, pull the rope and the hook to tighten.
Repair is now complete.

[edit] Warnings

Use common sense, do not use tow line if line is beyond repair.
Take care when cutting with sharp blades.
Consider why your first rope broke in the first place. If the rope snapped under pressure, the rope is now so stressed and damaged that it should be retired. If on the other hand the rope broke because it was sliced accidentally over a sharp edge, the rope may be strong enough to keep using.
Use a repaired tow rope only in an emergency. Always replace a broken rope when possible.
Use a repaired tow rope at your own risk.
Some insurance companies will not cover damages caused when using any tow rope. Beware.
Recovering a stuck vehicle is very different from towing a vehicle that is not stuck. The forces exerted on the rope are much more severe. Ropes and their connection points have been known to break loose and fly through windshields, injuring vehicle occupants and bystanders.
Lay a heavy blanket across a tow rope to slow it down if the rope breaks.