My Prusa thermister line broke as well, several months ago. The reason: they used solid (not stranded) wire from the controller to the thermister. Metal fatigue took its toll and the wire broke about half way between head and controller box.
I recommend running new twisted wire all the way from the box to the head. The Adafruit 30awg silicone coated multi-strand wire is nice and flexible.
I did a staggered cut on the thermister lead wire pair a couple inches from the head and soldered in new wire from that point (used heat shrink tubing, too).
It’s pretty easy to remove the few zipties and unwrap the wire shroud to gain access to the wire bundle. Splicing in removes the need to get into the head plastic parts to replace the whole thing.