Sorry Suzanne, it is unfortunate situation!
If your orders are over the phone, you could enter the order in Shopify, then charge the customer directly on authorize.net virtual terminal.
I know not ideally but better than nothing. You can then mark the order paid on Shopify.
Everyone needs to call authorize.net and ask them to escalate and get them to work with Shopify to take them back. It has affected thousands of people and tons of smaller apps too that only worked with gateways!
Do you have your own website or you are only using Shopify? I suggest you get your own, even a smaller one who is.not going to kick you out unannounced.
We have our own website and use Shopify for the shopping cart, but also have a backup shopping site with another little provider. Just found out you can even connect authorize.net tool directly to your website for the shop page.
Hope you find a solution soon!