Restringing a vertical blind is necessary when the string becomes frayed or broken. The string that runs through a vertical blind headrail needs to be in good condition for it to flow through the many guides and pulleys which exist inside. Fix My Blinds has several helps when it comes to this type of repair. Here are a couple video’s:
We also have several diagrams which show different examples of how string is routed through a headrail. Those diagrams can be found in this another blog post here.