This has happened twice already this year, and I always forget what to do to fix the problem!
Our sites “appear” to go down, really the browser just complaining that the site is unsafe, but nonetheless, needs to get fixed immediately.
Takes me about an hour to remember what to do, thankfully, in this case, it just happened to another site last week, so the information is still fresh.
So what exactly did I do?
Check to see if the certificate was still good. In our case, it is set to auto-renew.
Navigate to the service that needs the updated SSL certificate, in my case, it was an App Service.
Select SSL settings and then click Import App Service Certificate. The App Service Certificate Picker panel appears on the right.
Select the correct certificate and you are now ready to bind it your site.
Click Add binding. The Add SSL Binding panel appears on the right.
Select your hostname and appropriate certificate and then click Add Binding.
That’s it! Your certificate is now bound to your domain. The changes should take place almost immediately.
As of July 30, 2018, according to Microsoft Support, there is no way to have this happen automatically, hopefully they will remedy this in the future, in the meantime I would recommend that you add an annual event to your calendar so that next year, you won’t be scrambling like I did.