In rare situations, you may see an error message along the lines of "You do not have any template sets!" or "You do not have any stylesheets!". First, check with your FTP client to see if you have a .css file in templates/styles/. Then check if you have a subdirectory of templates/ which contains a lot of .tpl files within it (normally /templates/default or /templates/multilingual). If these are missing, they have somehow been deleted and will need to be reuploaded.

The most likely scenario, however, is a permissions problem. Check yoursite/servertest.php, and if it says "Apache mode" anywhere on that page you should bulk-CHMOD all files and subdirectories to 777 (if your FTP client doesn't do recursive CHMODs, try FileZilla). If the servertest.php doesn't say apache mode, then you'll need to make sure that directories are CHMODed to 755 instead and files to 644 (755 permisisons on files may work on some servers but not others).

Another possibility is that you're upgrading from an extremely old version of WSN, and haven't overwritten templates as required by the upgrade process in docs/readme.html.