Can't access admin: "you have no web pages" but site works

I am not sure what might have caused such a problem (especially knowing that everything has been working OK up to now).
Has your web host changed anything lately (such as PHP configuration settings) which might have caused the problem?

Unless something has happened with your web server, it is unlikely that you will have lost any of your pages. (The files should not have been deleted unless you or your web host have deleted them.) Check your Showkase file structure to see if the folders corresponding to the pages you have created are still there (in your root Showkase directory).
As long as they are still there, I would take this opportunity to make a backup of your entire Showkase directory just in case anything else goes wrong.

First of all, try installing and running the Showkase Server Compatibility test. If your web host has changed anything which would result in your web server no longer having all the functionality required to run Showkase, the Server Compatibility Test should hopefully pick it up.

If all tests pass, then try reinstating all the core Showkase files as if you were upgrading to a new version.
Instructions on how to do this can be found in the 'How to Upgrade' section on the Upgrading Showkase support page.

Hopefully this will resolve your problem.

As far as I am aware, the Hierarchy Request Error is a PHP DOMDocument error which occurs when trying to move a node into itself, although I do not know why this error is generated when you try to publish your gallery.

We know that Showkase works OK on your web server (as you have a second working Showkase site) and we also know that all the core Showkase files are present and correct (as you have tried re-uploading them, overwriting the existing ones) so the problem may lie with one (or more) of your Showkase pages. With this in mind, you might like to try the following.

*** Please make a full backup of your entire Showkase site before proceeding. ***

Try trashing your Showkase pages on the 'Pages' tab, one by one (starting with the last one you created and working backwards), and try to Publish your site each time to see if the Hierarchy Request Error still occurs.
(When trashing a page, it will be moved into the '_trash' directory. You can reinstate a trashed page using the 'Site -> Import' functionality.)
If you find a page which is causing the problem, you could recreate it manually and reinstate all others using the 'Site -> Import' functionality.

Unfortunately, I do not know what may be causing your problem but a possible workaround would be to create a completely new Showkase site and use the 'Site -> Import' functionality to import all existing pages from the currently broken Showkase site. This might be a relatively quick option to get your site up and running again without having to create all your pages from scratch.

Just to check, are both Showkase sites in completely separate directories? (I would not recommend nesting one Showkase directory within another Showkase directory.)

Steven,  the import trick is working: I set up a new folder on the server, uploaded the showkase files, then used "Import" to scan the non-working site, selected 9 pages for import.  Once done, my admin function is back, though, of course, I had to revise the nav order to get the links right, and still have lots of tweaking to do on layout, thumbs, button bar, etc.  STILL, the admin function is back, and the problem seems fixed.  THANKS.