Customer orders not showing up

I apologize in advance, I am by no means a web site developer, I have to google tutorials for just about everything I need to do on the back end of my website. With that being said...I run version v1.5.1 and I use Authorize.net SIM for my payment module. Since 6/26/19 my customers willtry to place an order and it tells them their credit card can not be processed but it charges them anyway. I will receive a settlement report email from Authorize.net showing a transaction and that transaction does show up in my bank account. However, my customer receives nothing from my website confirming an order or confirming a payment. I also do not receive an order confirmation from my website nor does an order show up under the "orders" tab on my site. I've called Authorize.net and they said the only change they've made recently was with the MD5 Hash, which was right around the same time frame as my problems starting. Any help would be greatly appreciated, but be kind, I am green!

Thank You!!!