It sounds like there is some code error in your new template files, or perhaps a file corrupted or missing, that is terminating output after the header. We may be able to tell more about your problem if we can see it live.

Newer versions of Zen Cart have a built-in error logging feature, but yours doesn't unless you specially installed it. Yet another reason to upgrade now, before you go customizing templates. Your products, customers and similar data will be unaffected by upgrading.