Fatal error in the output generally indicates either your theme doesn’t like 3.1, you have a plugin that doesn’t like 3.1, or you have an incomplete upgrade.
So, to start troublrshooting, you may try (if you already have not tried) the following troubleshooting steps and see if any one of them resolves the issue :
-deactivating ALL plugins temporarily to narrow down and possibly fix the problem. If the problem goes away, activate them individually (one-by-one) to find the problematic plugin.
If you don’t have access to your admin area, try deactivating them manually via FTP or phpMyAdmin.
-switching to the default theme (“twentyten”) for a moment by renaming your current theme’s folder in wp-content/themes. The idea is to force WordPress to fall back to the default theme to rule out any theme-specific issue.
If you don’t have access to your admin area, then access your server via FTP or SFTP or whatever file management application your host provides.
– resetting the plugins folder by FTP or phpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems (because the hooks remain unless plugins completely removed).
-If the above steps do not resolve the issue, download WordPress again and replace your copies of everything EXCEPT the wp-config.php file and the /wp-content/ directory with fresh copies. This should replace your core files without changing your content and settings in wp-config.php file and the /wp-content/ directory.
After you are done uploading the new files, point your browser to /wp-admin/upgrade.php
