There are several known sorts of reasons the screen might be blank:

Cause: You've just upgraded to a new version, but fields couldn't be added properly, and each category has become a blank page.
Solution: You probably have a broken mysql table index. Use phpmyadmin to look at the structure of your wsnlistings_links table. At the bottom is a list of indexes. If 'wsnindex' lists the same field twice, delete it. Then re-run upgrade.php.

Cause: File permissions problems.
Solution: If PHP and FTP run as the same user on the server, all directories should be CHMODed to 755 and their files to 644. Having files or directories with too many permissions can cause blank pages in some server configurations.

Cause: You've disabled the display of errors. If you set debug mode to "supress all errors" the screen won't give you any clue of what went wrong.
Solution: Set the debug mode back to normal. Use phpmyadmin to run the query
UPDATE wsnlinks_settings SET content=0 WHERE name='debug';
but make sure to change wsnlinks_ to the actual table prefix used by your installation.

Cause: The .htaccess file is broken.
Solution: Delete the .htaccess file, and turn off url rewriting at Admin -> Settings -> SEO if it's on. Then see the manual entry on url rewriting and/or contact your web host to determine if you can get that feature working again.

Cause: Check if you can see your site by going to index.php?debug=1 ... if you can, it would seem that somehow output caching has ceased working in your PHP installation.
Solution: Ask your web host to enable output caching in php.