1. make sure there’s a config.php or wp-config.php file in the root. The ugprade will have to edit this file.
2. make sure the database still exists, and that it still contains content.
3. check for database errors. These are generally found in logs, but how you access them will vary by hosting provider.
4. make sure index.php exists
5. try renaming or removing your theme in the actual files to deactivate the theme . If the theme is incompatible with the version of wordpress you’re upgrading to (many pre-version-3.0 themes will break if you try to install wordpress 3.2.1.). This will force wordpress to switch back to the default theme, but it’s the smallest odds approach.
6. If all else fails, try installing a test installation of wordpress with the most recent version your installer offers, in a separate directory or on a separate domain. Note that this will also require an additional MySQL database.
7. check your .htaccess file for issues
8. If you moved the site during the upgrade, either to a different domain or to a subdirectory or from a subdirectory, you’ll need to change the path manually in wp-config.php to be able to access it at the new location.