authorize.net returns blank page in checkout

[Black Ink]

I've configured authorize.net using the API login and transaction key provided to me.  When testing the shopping cart, I receive a blank page when submitting after entering the credit card information.  Both my bank and authorize.net tell me it's a shopping cart issue.  When testing alternative payment methods such as "payment on delivery"  the cart functions without a problem and the confirmation message is visible (no blank page).

Very frustrated.  Any help would be appreciated.

[Black Ink]

actually... now no payment methods work.   Anytime I submit an order for payment... it goes to the blank screen!  Ugh!