Broken blind strings will cause the blind slats to lie crooked, and will make it impossible to draw the blinds up or down evenly. Replacing a broken blind string is a simple process, but it does involve dismantling the blinds to remove the broken piece of cord. Once you replace it, the blinds will sit evenly and move up and down like normal.
Remove the blinds from the mounting brackets. Pull the end caps off the bottom and top rails of the blinds and set them aside. Pull off the cord lock from the top rail.
Slide out the top and bottom rail to expose the cords. Pull the broken string out through the top of the blinds, or the bottom, depending on where the break in the string is. Set the washers from the bottom of the cord to the side.
Cut the new blind string to the same length as the old cord.
Thread the new cord through a large needle. Insert the needle through the top cord hole in the top rail, then down through the holes in slats until you come out through the bottom. Remove the needle and thread the old washer over the end of the string. Tie a knot under the washer.
Replace the bottom rail, then the top rail. Pop the cap off the cord lock and string the new cord through the lock. Replace the cap, then snap the cord lock back into place on the top rail. Snap the end caps back onto the top and bottom rails.
This process works best on cellular shades and similar styles with thin strings, not on the thick cords of older wooden blinds.