Urgent problem with thank you page

[gkrehbiel]

Suddenly my customers are getting a "page not found" error on the thank you page -- that is, after they enter a credit card number and submit it to for payment.

The payment information is not getting through to our payment processing center, and no confirmation email is sent.

What kinds of things should I look for to diagnose / fix this?

Thanks,

Greg

Update: I temporarily allowed purchase order transactions and submitted a test. It went through fine. This implies that the problem is between my store and Chase Paymentech. I called them and they say there are no issues on their side. What else can I look for?

Update2: Now orders are coming through just fine. I have no idea what caused the problem or what fixed it. Is it possible that by adding and then removing the purchase order option I reset something?