Try this: https://www.answers.com/Q/How_do_you_fix_stuck_blinds_when_cord_does_not_release_the_blinds
Thank you, Holly and Cheryl. We had a cord stuck like that in a vacation rental, and the bldg. supt. the owners sent didn’t have a clue, so we were not able to open the balcony blinds at all.
It's not very difficult actually. You just need to open the system on top and add a new cord or the current one if long enough