If the string on your vertical blinds has come out of the loop mechanism or tensioner, repairing it may not be as difficult as it seems. Rollers and bearings inside the blind head rail and tensioner operate by running the string through them like a pulley system to operate the chain drive that opens and closes the blinds. If the string comes out of one of these devices, you can replace it by simply restringing it through the mechanism.
Measure the length of the string loop for your blind. If the cord broke, measure the entire string, and divide that in half to get the correct loop size. The loop measurement is the length of the intact string loop stretched taut. Purchase the correct replacement string loop size for your blind.
Remove the blinds from the brackets and lay on a flat surface. For large vertical blinds the size to cover a door, you may want to work on the floor to avoid damaging the vanes of the blinds. For smaller sets, you can lay them on a table to work. Be sure to pull the vanes to one side of the blind and lay them flat while you work. Alternatively, you can work while the blinds are in place; however, you must take extreme care that you don't put pressure on the blinds or pull them out of the brackets.
Remove the cap on the cord loop mechanism, or the end cap to the blind head rail where the mechanism sits. You may have to gently pry the cap off with a screwdriver, taking care not to scratch the paint or plastic.
Slide the old string out if it is still in the mechanism by rolling the gear forward until the string runs out.
Feed one section of the new string loop into the first tooth of the gear mechanism. Roll it forward, feeding more of the string into the teeth until you have moved in a full circle and have fed the string into the gear completely.
Replace the cap on the mechanism and rehang the blind if you took it down to work on it.
Bring the cord down to the tensioner device attached to the wall. If you do not have a tensioner device and the cord hangs on a weighted end, simply reattach the weight to the cord and let hang. If you have a tensioner, release the latch and slide the swivel cover over to the side. String the loop around the channel provided to guide the string when it is pulled. Snap the swivel cover back into place.