If you're getting a blank page when trying to browse to a certain page on your store, or after clicking to have it do something, you've almost certainly got a PHP error in one or more of your files.

Sometimes that happens because of failed FTP uploads where files are left incomplete.
Sometimes it's a result of an incorrect edit to a PHP file (perhaps an extra quotation mark in a define() statement).
Sometimes it's a result of certain addons being written poorly.
Sometimes it's because you've uploaded some addon files to the wrong folder.

There's a handy tool available to log these errors. It can be found by clicking on the Debug Logging Tool link in step 2a of this FAQ article: http://tutorials.zen-cart.com/index.php?article=82

If you're getting blank pages, use the debug logging tool, and then search the FAQ area and/or the Forum for the details of the errors you find in the logs. And it's always best to start with the first error messages first, since other errors are often a result of earlier ones.

FAQ on debugging blank pages: http://tutorials.zen-cart.com/index.php?article=82