You changed your PHP-Nuke theme to one that is buggy. Now, you cannot see anything, or you only see errors and the Administration panel cannot be reached to change
the theme - a chicken-and-egg problem...
There are various solutions to this problem:
Copy the following code into a file and save it as fixtheme.php:
If you don't have the NukeNews theme uploaded in your site then change the value for $def_theme in the code above so that it reflects one of your working themes, save it, upload it to where nuke's
config.php file is, then point your browser to http://yoursite.com/fixtheme.php (thanks to Humpa for this).
Just rename the DeepBlue theme (or some other theme known to work) folder to the problematic one.
Use phpMyAdmin and change the default_theme field in the nuke_config table.