I would say that you are taking a great risk with your boat and possibly yourself.
There is a reason the strand has broken, and unless you know definately why that strand has broken (like that you snipped it with a pair of pliers), the whole shroud is very suspect.


Standing rigging like shrouds, forestay and bridles are things you should change on a regular basis anyway. Some say every year, I find that a bit to often but you will have to decide for yourself.