I need to repair a stone wall capping as in the pics..Suggestions please as to

– Why did it fail in the first place?
– Can I resurface ( if so minimum depth of new ) or do I need to completely remove all of the old?
– What mixture with what sand would you recommend?
– should I add frost protector?

Anything I’ve forgotten..any tips please?

frost has got under it, it happens. make a mix at 3 to 1 sharp sand to cement, wet enough to form a ball but dry enough that it doesnt stick to your glove. take it back level and redo the whole haunching, including as much stone as you can. use a big car sponge to get tool marks etc out of the finish. wet it down every 12hrs after you finish, for three days. it is probably too cold at the moment though, but you should get away with it if you wrap it up in tarps for a few days. im avoiding stonework this time of year it has to be 5 degrees + during the day and not below 0 at night for me
feel free to pm me if you get stuck i would offer to look but im guessing your oxford way by the stone 😉

edit; oh and that is not a dry stone wall