The errors that browsers give for expired SSL certificates is pretty dramatic, so I hope it hasn't deterred anyone! The site was still secure, it's just that the SSL cert hadn't renewed when it was meant to.
I've made changes to the renewal process so that it works with our server hardening, but I'm also switching monitoring companies, as the one we are using didn't pick up the SSL certificate problem, which would have meant I could have fixed it asap.