Order email invoices suddenly not coming through

we have run into a recent problem with orders going through Zen Cart and Authorize.net, after the order is finished we don't receive the order invoice and cannot follow up on shipping the order. We have as of late received emails from our customers asking for their orders and we cannot follow up nor even confirm the items they had ordered which is very distressing. Please Advise!

That suggests that something is breaking the process which occurs after the payment is accepted.
Have you tried a checkout yourself to see what exactly is actually happening? When was the last time you went thru checkout yourself?
What's showing in your site myDebug-xxxxx.log files? https://www.zen-cart.com/tutorials/index.php?article=82
Which authnet module are you using?
When did this start happening? How does that coincide with ANYTHING related to your site? addons? upgrades? changes in your admin by any staff? etc
What version of Zen Cart? PHP? MySQL?
Who's your host.
And any other questions mentioned in the "Posting Tips" section of the screen after you click the Reply button below.