on our bedroom window with faulty strings we just replaced the day/night shades with a roller shade. I'd bought the string to replace but decided we wanted to just go to a roller since we rarely open the window and the bed sits under it making the bottom hardware get into the pillows.
2010 Winnebago Vista 32K with a 2013 Mini Cooper S Roadster toad.
Short of replacing the blind, take a close look at how the strings are routed through the metal slats. To prevent wear on the string (and breakage) there are small plastic inserts in the metal slats. The cause of strings breaking is wear on the hole in the metal slat. The plastic insert prevents that.
replaced all our blinds with Levolar Convertible Blinds purchased at Lowes. They cut them to width when you purchase them, brackets are included with the blinds and they are not too difficult to hang. We used "light filtering" for the living area and "room darkening" for the bedroom. They work similar to the existing blinds but no more strings!