As serp belts can be inspected every time the bonnet is opened and even if found there's some wear and showing cracks in the rubber they will still last a few thousand + miles, so why did yours snap?
Hearing about them breaking is a first for me in my 19 years of interest in a "serpentine" belt among other things, only because my disco has one, they do appear to harden and cracks start to appear, so time to think about replacement sometime maybe, then it only takes 10 minutes to replace which I did once... years ago.