Hold onto the cord and pull it high and to the far left to see if that releases the locking mechanism. If not, pull it high and to the far right. Sometimes the lock can jam, and it only takes a little extra pressure to release it. If the blind now functions, nothing else needs to be done.
Remove the blinds from the mounting brackets by either releasing the clips there or by unscrewing them, depending on the model. Grasp the headrail with both hands and pull it out of the brackets. Place the blinds on the floor.
Push the cord out of the way so you can look into the small opening where the cord enters the headrail. Peek inside to identify a small metal lever. On some models, you might see a small metal knob.
Insert a flat-head screwdriver, needle-nose pliers or other long, thin object to push the lever or knob. This releases the locking mechanism and allows the cord to travel through it again.
Hang the blinds back up and test the pull cord a few times to ensure the locking mechanism is working properly.