Authorize.net working fine...Now Errors?

[blscott]

Authorize.net was working fine today and orders were being placed, then my customers started getting a blank page on checkout.

I am also getting the following errors when trying to check out:

Error: Couldn't resolve host 'secure.authorize.net'
Error: Failure in Processing the Payment (ps_authorize)

I have verified that the API Login ID and Transaction Key are correct and even changed the Transaction Key, but still get the same errors.

I have not made any changes to the root, so not sure what the problem is.  I also got a popup once that asked if I wanted to download: index.php during checkout.  Can anyone advise a fix?

Thanks,