We have Irvine Shades & Door pleated shades and one of the has lost all tension and will not stay up.
It is just a pleated shade with four strings , not a day/night or box type.
Cannot find anything on UTube to help.
Fix my Blinds www.fixmyblinds.com/Blind-Repair/Parts has detailed instructions for many blinds. They sell the string in several colors. Pay very close attention when taking your blind/shade apart, and have lots of patience.