Is the cord broken or just jammed. If it is jammed start at one end and follow the cord until you find the jam .
Usually a jam is caused by one slat being off angle or twisted. Follow what Unique Creations suggests.
You can open the top of the blind up and unjam the string or actually replace the whole string if it's broken or damaged. We do it all the time.
Sometimes the string gets caught over the end slat.