Customers can't get past the billing page.

On my ecommerce site the customers can no longer get past the billing page.

https://thebudgetgraph.com/site/index.php?main_page=no_account

I have tried filling in all the information but when I click "continue checkout" the billing page just reloads and all the fields remain the same and filled out. I'm not sure what is going on.

The only changes I have made redcently are changes to some of the title tags in a shipping module,
http://www.zen-cart.com/forum/showthread.php?t=75981

Please help. fill out some nonsense data in the fields and see for yourself.

And you have some type of add-on for your site?

Note: your secure certificate is for:
https://www.thebudgetgraph.com

And your site appears to be configured to use:
thebudgetgraph.com

i figured it out,

when inputing data that is already in the customer database it forces you rto 'login' rather than repeat the billing process.