A month or so ago, I downloaded the sample app solution for Visual Studio, from 

https://github.com/AuthorizeNet/sample-code-csharp (I needed to use it to update some custom C# code to use TLS 1.2). 

I was able to run the method: ChargeCreditCard

However, this weekend when I tried running that same method, I received the error:

System.Net.WebException The request was aborted: Could not create SSL/TLS secure channel.

I can't think of anything that's since changed on my computer (I did try with the firewall disabled, thinking that was interfering with the communication, but that made no difference). 

Any ideas on what could have changed? Help! (Need to upgrade my custom code to TLS 1.2 ASAP).