The hypothesis that I am working with is that your database and code are out of sync. But before giving advice, it would be helpful to be able to confirm this, hence the request for the date on the stylesheet and for info about the database prefixes. This would still be helpful if you have it.

Longer term, the safest course of action will be to upgrade to the latest version (1.3.7) of Zen Cart - after all that's the one with the most security fixes in. But that's a pretty big undertaking as you would be crossing from 1.2.x to 1.3.x and there were some significant structural changes in Zen Cart with this transition. So some idea of what changes you had made to your Zen Cart and how extensive they were (stylesheet, template files, module file, language files, extra functions, etc.) would help in giving advice.

And as always ... a live link to the broken site can be invaluable.