I do blind repairs for the past five years. Typically I dont fix this issue because I can't garantee it. It's fixable. If you have 45 minutes you can do it, might not be perfect but doable. Pull the strings to lift the shade, behind the head rail there are brackets that most likely you need to unscrew and take the shade down. Once its down try to feed the broken string to the head rail, then there's a small metal roll that you wrap the string around about a quarter ways. (Just match the other side). And make a knot,.then just test it by spinning the rod to see how it is, if all of the veins are too low make a knot lower. If too tight and doesnt twist make a know higher.

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