Half my products suddenly get "Page Not Found" errors

My version is 1.3.8a, PHP Version 5.2.17 (I know that I should upgrade my Zen Cart, but all the customizations and graphic layouts were installed in the template_default folder, and we're afraid that we would lose all our customization and that everything would revert back to the Zen Cart default look if we upgraded.) The web site has been running for a number of years, mostly without incident.

Anyway, this is my issue:

About half of the product pages of my web site have suddenly disappeared. We did nothing unusual besides updating one of the products. All the products do appear in the product listings on the web site, but once you click on the product link, you get a Page Not Found error for many of them.

In the Zen Cart admin, everything appears as normal. When I try to repost one of the affected products, it seems to go through successfully in the admin. But the affected product still fails to appear on the web site.

In researching this, one possible suggestion was to restore the includes folder. I checked and I don't have any backup available of the site.

I also tried the Debug Error Logging Utility, but no debug files have been produced.

Here are the links for you to see:

Example Product Listing (all the products are listed OK)

But clicking on any of the links brings a blank page.

Another Example Product Listing.

Clicking on the first two products leads to a blank page, while the rest are apparently OK.

It's a pretty big error, since our main products are the ones affected, and there are so many products affected that the site is basically useless as is. Please advise, and thank you for any help.

Possibly an issue with your URL rewriter. Can you turn it off temporarily?

I should also say that your template is just the Classic template with some colors changed. I would urge you to upgrade and apply the stylesheets from your current template afterwards. You're using the Classic template now.

You're a genius. I turned off the Ceon URI Mapping plug-in, and it worked. All the products are now available. Thanks enormously.

Now that I have that fire put out, I'm going to see if there's an update for that plug-in. And I know that I need to set aside some time and get that site upgraded. One of my other main concerns with upgrading is losing all the Spanish-language information that we've put in. Some time ago, we added a plug-in that allowed us to have the site in both English and Spanish, and I'm afraid that upgrading will make us lose all that translated text. Is that something I should be concerned about, or will an upgrade not affect the Spanish-language content?

Thanks again Stevesh; you've been an enormous help.

I'd forget about the ceon mod and upgrade ASAP. Many here, including me, believe that URL rewriters are more trouble than they're worth.

Language packs consist of mostly language (natch) and some template files. Download this http://www.zen-cart.com/downloads.php?do=file&id=1110 for your 1.5.1 site. The translated text in your product descriptions, etc. is stored in the database and won't be lost as long as you upgrade the DB properly. Backup first.

And please don't make this mistake again: I don't have any backup available of the site.

I'll definitely make sure we have backups and I'll set aside some time soon to upgrade the site. Again, thank you greatly for your advice, which has been spot-on.