Continue to checkout not working

My site was up and running with no problems and then someone contacted me yesterday to tell me they couldn't complete the checkout. After you are on the shipping page and fill that info in when you click on the "continue to payment" button it just goes to a blank page. Looking at the navigation it appears that it just goes to some blank page titled index.php?main_page=checkout_shipping which is the same page that it starts on but now it's blank. I'm thinking that link isn't working as it should be going to the " index.php?main_page=checkout_payment ". I've tried to find where this is for the last hour and can't seem to get this fixed. Can you help me?

Also if it helps my website is https://www.justspice.org

https://www.zen-cart.com/tutorials/index.php?article=82

Thank you for directing me to that info. Would you happen to know exactly what page I should look at that would be effecting specifically the page that gathers the customer and shipping information as it is the navigation from there that is giving me the blank page.

Thanks

I went in and downloaded the debug thing a ma bob and got this error in the cache/ my debug log

[24-Nov-2010 21:17:46] PHP Warning: Cannot modify header information - headers already sent by (output started at /nfs/c05/h03/mnt/76286/domains/justspice.org/html/includes/languages/english/checkout_shipping.php:70) in /nfs/c05/h03/mnt/76286/domains/justspice.org/html/includes/functions/functions_general.php on line 44

Could someone please tell me what I need to change on line 44
Thanks

It's not line 44 of that file. It's line 70 of the file where "output started at". Explained here in this other FAQ article:
https://www.zen-cart.com/tutorials/index.php?article=87

(FAQs are a handy reference to use)

Thank You. Got it!